OldMan senses something amiss with the rat and despite hearing the commotion from the crevasse.  OldMan decides to cast frostbite 18+6=24 to hit 3 damage.

 (Galben turns and yells down the hall at Thallan, Regula and Bearkiller)

"Glasstaff is out this way! Come quick."

(Thallan, after hearing Jake makes his way to the end of the hallway, in front of Galben)

(Bearkiller hearing the shout moves out of the common room towards the hallway where the door that leads to Droop is, heading down that hall towards the opening of the tunnel to gain a different vantage point)

Jake's attack hits. As soon as Jake attacks, Glasstaff quickly quaffs a potion of invisibility and immediately disappears. Jake moves across the bridge to try to block the wizard but not being able to see exactly where the wizard may have moved is unsure where exactly to stand, so his movement is an approximation.

As OldMan's attack hits the rat, you hear a loud squeal from it and collapses to the ground, dead. When it dies, it simply disappears. OldMan would recognize that this rat was, in fact, a familiar.

Question Can Jake's detect magic allow him to see a magically invisible person?

OldMan quickly moves over to the door on the east side of the room and quickly checks for traps and locks.  3+6=9.  I'm assuming I didn't find anything.

Jake can sense if the magic is within 30 feet of him but only sees an aura if the object or target is visible. So, in this case he can only generally tell where Glasstaff may be and he felt the magical presence leaving his radius to his south as he crossed the bridge.

OldMan does not find any traps on the door to the east from the workshop.

(Bearkiller continues through the hallway to the opening of the tunnel, yelling at Jake)

"What's going on?"

(Thallan moves onto the bridge)

(Galben moves into the main room and moves down the left ledge and stands there)

"Jake where is he?"

Tubard runs towards the entrance of the cave to try and block it.

Jake swings wildly at Glasstaff as he passes by.  (13 to hit, usually disadvantage due to invisibility, but Guiding Bolt makes it a straight roll.)  13 to hit, if it hits, 7 damage.

Jake's attack hits nothing but air. You do not sense the magical presence within your range any longer.

(Bearkiller makes it to the opening of the tunnel, standing in front of it, looking for instruction for the next move.)

 Jake looks to the others to his west.  "I believe he is south of me! he probably went to the east, down one of those tunnels!  Who here is effective at tracking?"  

(Galben crosses the Bridge and looks around the ground and walls for clues of where Glass staff may have gone. Perception roll 15+4=19)

(Thallan moves towards the entrance of the tunnel and looks at Bearkiller)

"I'll guard this entrance, you go help the others." (Thallan plants himself in front of the tunnel leading out)

(Bearkiller moves towards the north bridge)

OldMan is still in the lab.  It seems like maybe Regula is still there.  "Regula what say you?  I don't like splitting our group up but I feel like we should go in the next room.  I can't do that alone.  If you think we should go rejoin the others I will."

Galben does not see any evidence of Glasstaff in his search. There is no sound or stirring other than those created by the party.

Regula looks at OldMan. "Yeah, I think we can get to the bottom of this. Let's look at the other room and see if there are any clues to be found."

Regula and OldMan enter the next room. The walls of this bedchamber are covered with drapes of scarlet cloth. The furnishings include a small writing desk with matching chair, a comfortable-looking bed, and a wooden chest at the foot of the bed.

OldMan quickly searches the room for anything of use and anything that might be hidden or any secret doors etc.  Investigation roll 14+6=20 

"Regula it makes sense that this is Glasstaff's quarters off the lab.  I would venture a guess that he has a spell book and other things in here.  Also there may be a secret door.  Glasstaff seems like the kind of person that would keep an escape route always."

Jake curses loudly at losing Glasstaff.  "Should we give chase?  Or just proceed upstairs and Duke it out with him?"

Jake then goes into the laboratory, scanning with his detect magic for anything hidden, or obviously magical.

OldMan hearing Jake in the laboratory calls to him.  "When you get done searching in there come check this room.  I sense this is the wizard's sleeping quarters and he may have things hidden in here."

Jake follows OldMan's voice into the sleeping quarters. "Ah, the head Ruffian's quarters.  I got him with a Guiding Bolt, but he was too slippery for me.  He drank a potion and disappeared. I suspect he fled upstairs and he may have reinforcements.  We should press our advantage and assault before he regains more strength."