The Firehouse Perimeter Alert is a security device designed to alert the Ghostbusters of unwanted intruders coming in from the front door.
In November 1997, Lillith took it upon herself to arm the Firehouse with a perimeter alert. Any unauthorized visitors would set off the alarm and could only be shut down by a remote control. Janine Melnitz was not pleased repeatedly set it off. Eventually, she blasted it with an older model Proton Pack.