"What the hell else in down here?"

"Do you think we should risk a peek up to orient ourselves?" asks Oriace trying to figure out if the symbol indicates that they should continue down or head up.

The symbol is past the ladder. It's similar in placement to the first one you saw when you came down a ladder, positioned so that people coming down will know which way to go without guessing between a "from" and "to" of your path 

Sterling gingerly stepped past the fallen creature. "No telling where the ladder leads," he said, "or if its guarded at the top. It's probably safer to just keep following the symbols."

"Yeah, keep following..."

“All righty, let’s keep going. I just thought if we have to run out then it would be good to know what’s up there.”

After 5 minutes, the main sewer tunnel expands into a circular hub with a pair of arrow slits carved into its outer walls, directly across from each other. Two passages continue on to the north and south. A stone door is set into the back wall of a stone ledge to the west. 

Map (Blue circle is approx party location, also for scale, the circle is about 5ft (one square on map), and north is up.):

"Defensive structures? Not good. Hmmm."

Sterling examined the arrow slits for a moment, then cast a spell to conjure an illusory wooden board over the slit opposite their entrance. With his chances of being shot now halved (he hoped,) the bard cautiously made his way to the stone door and examined it.

(( Investigation Check: 10, Perception Check: 9 ))

Oriace dodges and follows Sterling to investigate the door. On the way to the door he peeks north and south the corridor and observes if there are any pressure plates on the floor that might activate a trap. "This looks like a trap to me. But good call, Sterling."

(Perception: 16)

I am going to assume that you are all sensible, and that you aren't shouting to each other, and are more likely whispering when you talk, seeing as you don't know who is around.

The door is not trapped, and is not locked. Oriace, you do notice something on the south corridor. There is a secret door just opposite the south end of the ledge. 

Oriace points to the segment of the wall where he spotted something that looks like a secret compartment or door. "Perhaps that leads to a chamber behind the murder hole?" he communicates in silent speech (telepathy) to the others. "I'd say we check that one out first," he says and approaches the secret door and tries to open it slightly for a peek inside.

It is also not locked or trapped, and opening it you see a short corridor (10ft long from memory) before another door. The north (left) wall of the corridor has another arrow slit in it.

I will update the map once you move somewhere more significant than a tiny corridor.

Looking at the arrow slit, Wagner says, "I can get through that. it'd be a hell of a surprise to whoever is on the other side, but will likely end in my bloody death. What do you guys think?"

Sterling grimaced. It was definitely worth a shot; scoping the position out before they all rushed in was wise. But if there was trouble...

"It seems a bit risky. But I can't say it's a bad idea. If you're willing to go for it, the door's not locked; we can run in if there's any commotion."

"Don't you think they would have shot at us already if anyone were inside?" asks Oriace wondering. "If you think it is dangerous, you could let an illusion enter before you do, and see if it causes any reaction," he suggests.

There have been plans in the OOC thread. But until they are posted here, time is effectively moving very slowly Nope, that is the warlocks, who are also debating what to do but getting into the warehouse as they started after you guys. But still applies of I need an actual action rather than just a discussion. Renaer doesn't have any input into this. He is following you guys ready to fight if needed. But he is not an adventurer so the plans are up to you all

Wagner peers through the arrow slit and uses misty stepto teleport to the other side of the wall.

When Oriace sees Wagner teleport inside the chamber, he hurries through the secret door and the door just behind it, hoping to find Wagner inside.

"Wait a... ah, crumbs!"

Sterling hurried after the others.

Wagner appears and sees a sleeping goblin, who was supposed to be on watch. The other two of you and Renear find yourselves in a larger room not with Wagner. In the middle of this otherwise empty room is a stone pillar carved with a small symbol: a perfect circle with ten equidistant spokes radiating outward from its circumference. In the middle of this circle is a smaller circular indentation that bears a passing resemblance to a lidless eye.

Map: (circle majority of you, room above Wagner)