hmm...that's pretty odd. It has nothing to do with car audio so i wouldn't really know about it. All i can suggest that that yea it comes on when like ur doors are open right? So my guess is that most likely the switch that keeps that beeping noise from beeping malfunctions. Don't really know how to replace it or to even check if it's that.
What kind of car you got? Because if it is nothing than just a screwed up switch, you could disconnect it so that it'll NEVER make a noise again! or unless u reconnect it.