Why would my Pirhana alarm go off whilst I'm driving?

My Pirahna alarm has started going off whilst I'm driving.I can stop it by pressing the 'lock the car' button on the remote, which is hanging from the key in the ignition. It seems to have started since the battery went totally flat and I disconnected the battery to recharge it. The remote locks the car and sets the alarm ok, and it doesn't set itself off whilst the car is locked and stationary. Don't know which model the alarm is but it's fitted to a 1994 car probably as a retro-fit, so later than 94. The fob is black rectangle with 12v battery power and a black button to lock/unlock and a red button to set the alarm off manually if necessary. Any help gratefully received.