If its the factory alarm (pathetic beeping from the little alarm horn under front pan controlled by door key switch), see this thread. This solved my problem for nearly three years now on my 81. Problem simply related to age/corrosion. I too had no key and never messed with it until it randomly sounded. See top post and resolution at the bottom. Hope it helps.
http://forums.pelicanparts.com/showthread.php?t=289240