You will see the emergency exit when you turn left at that corridor.
When you think of a place where bad high schoolers skip class and hang out, it would be under the emergency exit staircase, right?
After putting Aya’s drawing under his coat, the thief slipped out through the emergency exit.
You know how 非常 means emergency and how 口 can be some kind of doorway (like in 入り口, for example)? Well, if that's the case, then you have an emergency doorway. Usually you run away from emergencies, though, making this an emergency exit.
The reading is the words 非常 and 口 combined together. The thing that makes this weird is that it's a mix of the kun and on'yomi readings. I guess things get all jostled up when you have yourself an emergency.