Before he can answer (with something other than, essentially, "You crazy?") the police dogs start barking because they've picked up a scent. They're scrabbling at a door that leads to the upper floors, and the doorman tells them there aren't any building exits on the floors, which means he could still be up there. Broyles orders the building locked down -- that wasn't already done -- and gets on the radio to order every available unit to Elmwood Apartments.
After the commercial break, it's dark, there are more Boston Police dogs going nuts outside, and an agent tells Broyles that the last floor is evacuating, and the residents have been instructed to leave their doors open. Broyles orders everything shut down except the elevators, and the building goes dark.
Agents make their way inside shining UV lights everywhere. Olivia's taking her team up to the highest floor and working down, while Lincoln's going to the parking garage and basement and working his way up. They'll meet in the middle.
So the agents go searching, and we get lots of atmospheric flashing blue lights, and after searching ONE FLOOR, Olivia decides, as they're heading down the stairs to the next floor, that this is taking too long, and says they should split up. She'll take this floor, and she sends the other agents down to the next one. Uh, I know you have UV light on your side, but you're assuming that Walter's mouse experiment will translate to a human (I mean, I know he's always eventually right, but STILL) and might I remind you that you're searching for a serial killer WHO IS INVISIBLE so maybe the buddy system is the best way to go?
But no, Olivia's going to wander through the dark alone with her UV flashlight. She finds herself in a part of the floor that seems to be undergoing renovations -- and she falls into some sort of hole. It's too small to be an elevator shaft -- some sort of heating/electrical access or something? Whatever it is, it's deep, and Olivia barely manages to catch herself on the lip. See, Olivia? BUDDY SYSTEM.
Meanwhile, the agents down below her are following the dogs, which have caught a scent, but it just leads to a discarded shirt, which they classify as a "false alarm" because yeah, what would a discarded shirt have to do with a guy who can turn his body invisible? At this point you have to wonder if a future Fringe plot will involve someone adding stupid pills to the Boston city reservoir.
Anyway, Olivia's struggling to lift herself up, when Eugene slowly comes into range of the UV. He looks all creepy and ghostlike under the UV. Actually, he reminds me like nothing so much as Michael Myers in his mask, with the black eyes.