Chirping while off, Camry

Car makes chirping noise (similar to the 'beep' it
makes when fob is used to lock the car) when off,
parked, with doors closed and no key/fob left
inside.
I only noticed this recently, mentioned it to my
neighbor who says she has heard it many times
while outside in the past few weeks, and says it
persists for several hours beeping every few mins.
I worry about the battery for one, but I've tried
securing all doors/drunk/gas cap, looking for
alternate sources (no garage opener or anything in
car). I dont want to go see a mechanic for
something silly, but I have to stop this noise!