Wagner drops invisibility in a flashy way with a flourish  to conceal that he was actually hiding.

"What's your name young man?" Toser asks the boy.  "That is an important piece of evidence you have there.  We must turn it over to the watch.  I'll stay with you for the moment."  Toser then waves to one of the investigators and beckons him over.

((Assuming someone comes over))

Addressing who comes over he introduces himself. "My name is Toser and I am one of the proprieters of the Trollskull Manor here on Trollskull Alley.  This young man has something very important that he found. He wishes to turn it over to you." he then turns to the boy, "Go ahead, its ok" he says reassuringly.

Disappointed by the lack of evidence uncovered at the Bent Nail, and hoping to help give Oriace cover if needed, Sterling returned to the entrance to the manor.

"Sergeant," he nodded as he approached. "I'm Sterling, one of the proprietors of the manor. How can we help you?"

Oriace sends a quick telepathic message to Sterling, his tone calm but firm: "Say that I'm out buying supplies. Heading into sewers. Follow asap."

Then, he pivots away from the main road, keeping his movements slow and natural to avoid suspicion. As soon as he finds a moment where the sergeant’s attention is diverted—perhaps due to the noise he had caused earlier—he silently makes his way behind the Bent Nail, towards the sewer access.

Once near the grate or entrance, he carefully checks for signs of recent use—disturbed dirt, fresh scuff marks, or anything indicating that someone had fled underground. (Investigation: 21; Perception 5) With one last glance to ensure he is unobserved, he prepares to slip inside, vanishing into the shadows. 

[[I’m gonna play this like they know me and have scolded me for my investigations in the past.]]

”Look, I know I’m not supposed to get involved anymore, but I took a look up on the rooftops where that woman saw someone, and I found the broken clasp of a necklace on that building there.”

Let's try and make this as simple as possible. 

Toser: the constable who comes over takes it, and delivers it straight to the mage with a little bit of a summary of the boy's (called Martem Trec) message

(good choice. Extract from the module: Characters who withhold the necklace, and knowledge of it, from the City Watch are guilty of a crime in Waterdeep. Hampering justice by concealing evidence can result in a fine of up to 200 gp and hard labor for up to a tenday.)

Oriace: Despite the obvious signs of the gnome having spent time in the sewers, it doesn't seem to have been here. There is no evidence you can see of this grate having been used in the last few days. There are guards posted at the entrance to Trollskull Alley due north of the sewer grate, who would probably notice the sewer grate being lifted if you tried to enter.

Everyone else (assuming Mana joins at the manor):

"I'm Seargent Cromley, however I'm here as a witness. Barnibus Blastwind has some questions for you."

"Indeed I do. I have more or less worked out what went on here. And I appreciate your help in finding the murder weapon. Unfortunately I am missing a key part of the puzzle that I hope you can help me with, and then we will be on our way, and you can leave the investigation alone. Who was the gnome?"

Mana is all ears. At the question of who the gnome was, he silently shakes his head, raises one shoulder and presses his lips together.

”I’ve never seen him before.”

His voice breaks, clearly still distraught by the wanton taking of life.

"Neither have I," shrugged Sterling. "But if you don't mind me asking, why the gnome? Can we presume you've identified the others, and he's now the primary suspect?"

The mage hesitates before answering (make a persuasion check) 

Oriace returns to the group just as the conversation with Sergeant Cromley and Barnibus Blastwind is unfolding. With a casual shrug, he excuses himself, his voice carrying just enough concern to sound natural. "I thought I heard something out back, but it turned out to be nothing—perhaps just the watch moving around near the northeast entrance."

He crosses his arms and exhales, his gaze drifting briefly toward the bodies before returning to the conversation. "As for the gnome, I’ve never seen him before in my life. A real tragedy, this. I can’t even begin to imagine the kind of impact this will have on the neighborhood… and on our business, for that matter. Trollskull Alley was supposed to be a quiet, respectable place. Now it’s a crime scene." Oriace shakes his head, a mix of frustration and sorrow evident in his expression.

Toser also has no knowledge of who the gnome is and let's the Sgt know.

"C'mon Barnibus, at least give us a little hint what you are thinking. I really tried to interfere as little as I could stand to this time..."

Persuaion: 26 (Game Log)

"I don't want you taking this information and trying to do your own investigation. You have been borderline lawbreaking in your investigations before, and we can't give you special treatment just because you have helped us. But seeing as your property seems to have been caught up in it, it is only fair to share with you my findings. The gnome was running from armed pursuers, of which there were three. But only two of them are amongst the dead. The gnome and his pursuers were moving towards your tavern which is why I was hoping you could identify him. He clearly had intention to come to you. Neither the gnome nor his pursuers saw the blast coming. This makes sense considering the necklace of fireballs that was found broken on the rooftops. They were focussed ahead of them and the roof was behind them and the sun was behind them making seeing an attacker up there much harder. I do not believe the necklace was dropped. Based on the clasp found, it was likely caught on something and broke off, and the attacker was too focussed on fleeting the scene to stop and reclaim it. 

If you have anything else you can add that you saw, it will hopefully help us narrow down what happened and find the person responsible as well as their motive. I do not think that your tavern is in any further danger, it seems targeted on that gnome and so as long as you truly do not know him, you should be safe."

Oriace furrows his brows, carefully weighing his words. He had no intention of revealing the black, winged snake tattoo he found on one of the pursuers—at least not yet. Instead, he leans forward slightly, his tone inquisitive yet measured.  "How do you know for certain that they were pursuers? Perhaps they were trying to protect him from something... and failed?"

He lets the words linger in the air, studying Barnibus for any reaction. Internally, Oriace turns over the puzzle in his mind. The Zhentarim. The black network had its hands in many things across Waterdeep, but why would a gnome, possibly carrying something valuable, be headed towards their tavern? Was he sent by the Zhentarim, or was he fleeing from them? Why did the third pursuer escape and what did he take? Certainly something he wasn't keen to share with the Watch, or any other faction. 

He resolves to contact Davil (was it?), the Zhentarim informant he knows—trusted or not, they might have some answers.

“I’ve sworn off the vigilantism that the streets, especially since my last mission got me overwhelmed and nearly killed. I just couldn’t help myself here , with it being so close to our place.  I hope your investigation goes well.”

“Thank you both for your service. I feel much better knowing you are ‘on the job’.”

Mana can’t help but wonder if the investigator’s summation redacted the automaton and direction of the fleeing pursuer on purpose or not. He turns, bites his lip to say no more and excuses himself to go find and comfort Yersinia

(( Persuasion Check: 11 ))

Sterling was just about to try to ask again when Wagner managed to get the investigators to open up.

"The gnome was headed to us??" he thought as the conversation continued. The merchant was stumped, since none of them seemed to know whom the gnome was. Then again, some of their group were part of factions that the others weren't, so maybe it had to do with the silly political maneuvers of the city? Oriace was the one who'd actually looked the body over--Sterling made a mental note to chat with him later.

"Well," he finally added, shaking his head in response to the request for more info, "in the absence of any direct connection of ours to the gnome, it seems like this third pursuer is now the crucial piece of the puzzle? Strange how he's managed to vanish right in the midst of everyone. Still, if we are safe, as you suggested, then I suppose we'd best let you get to finding him. Or... whatever it is you feel will be your next step. Have a good day, gentlemen."

"Simply put, I do not know for certain that they are pursuers. But from the evidence I have seen, it makes the most sense. If they were trying to protect him, then the third person would have surely stayed. Additionally, from the gait of the footsteps, it was clear that the gnome was running away from someone or something, plus he had his dagger drawn so must have felt threatened. While the three following him seem more calculated in their movements and less panicked. The third pursuer is key to this investigation, as is the mystery fireball flinger. I believe we have collected all we can from the crime scene now so we will be lifting the road blocks and you are free to go about your day. Thank you for your cooperation."

And they both turn and leave.

Now, we all know that you guys are probably not going to be happy leaving this investigation to the watch. So if there is anything you wish to do, now is your chance to tell me. Some activities might take a few days (like asking a contact to investigate on your behalf) so we are in semi downtime. You may also go to places together or split the party. 

Oriace waits until the watch is out of earshot before addressing the others in a hushed but firm voice. “I held back from saying this earlier, but one of the dead had a Black Network tattoo—the winged snake. That means at least one of the ‘pursuers’ was with the Zhentarim. If the other dead man was also Zhent, then someone just wiped out two of their own in broad daylight. That’s not something they’ll ignore.”

He pauses, considering their next move. “I don’t know why the gnome was coming to us. But if the Zhentarim sent him, Davil Starsong or Istrid Horn will know. And if they didn’t, well, they’re going to be very interested in figuring out who did. I say we go to the Yawning Portal and ask around. They might already be investigating, and if we show up before they come to us, it might work in our favor.” Oriace folds his arms. “If no one has any other leads to chase here in the alley, I’m leaving now. I can go alone, but if anyone wants to come, I won’t turn down the company.”

After the authorities leave, Mana listens to Oriace while retrieving Yersinia. He stoops, cups his hands for her to climb into, and returns to the group with Yersinia perched upon his shoulder. She seems to have calmed down now that she is with Mana.

”Could that gnome have been in league with Xanathar? The dried muck on his boots would indicate his passage through the guild’s venue, at least in the somewhat recent past.”

Mana realizes this was a bit of a nonsequitor, but his mind was churning a million miles a second. He shifts his train of thought back to something Oriace had said.

”Unless you did something to conceal the tattoo, Oriace, I’m sure Blastwind knows Zhent were the pursuers. It’s odd that the survivor possibly fled to the sewers to escape, especially if their quarry was a Xanathar operative.  Or maybe he looped around ‘the Nail’ and left the Alley via the Northeast entrance. … And what did he take from the gnome? Surely it was more valuable to him than the satchel of gems.”

”And who or what would kill all of them without discrimination?”

Mana pauses to catch his breath.

“I would go with you, Oriace.  Perhaps it is time to lean on some of our alliances for assistance, even though I did not personally align myself to the Zhent.”

Another pause.

“The Harper’s might have information and the clerics of the Emerald Enclave would have access to the bodies. I’ve heard there is magic that can be utilized to gain information from those whom have recently departed. We would just need to know the right questions to ask.”