Motion Detectors

["Ray" <nightwatchman94@xxxxxxxxxxx>]

What do ADT motion detectors look like and where are they usually
installed? I live in a 3-family house, and the first floor tenants have
an ADT alarm system. Whenever I leave or enter the hallway, there is a
beep or chirp sound coming from inside of their apartment. I suspect
that it's a motion detector. What annoys me about this is, whenever
this sounds goes off, they open the door to see who it is coming or
leaving the house. It's driving me nuts. I feel like I am in a prison
of some kind.

I looked all over the hallway, but I can't find any kind of motion
detector. Is it possible that the motion detector could be installed
inside of their apartment and still detect motion from in the hallway?
All of the motion detectors I have seen are usually white bulky boxes,
but I don't see anything like that in the hall...

Thanks