it's a low battery beep from the hidden cameras :-P That was a joke.
I'm guessing they have some technology that lets them know something by the frequency of the beeps. Maybe it's a timer and if it's a constant beep...they need to do something to reset it and "tidy up" the restroom.
Could it be a counter with each time the door is opened or a toilet is flushed and at a certain point...that beeping changes?
I think at @drdoggie00 should ask corporate BJ about the noise. Visit the corporate site and ask why you are hearing beeping in the bathroom and mention hidden cameras and see what kind of response you get.