An arbitrary car alarm

The alarm system in my 2001 Prius goes off at odd times, whether I’m standing beside it, upstairs asleep, in the grocery store, or wherever. I have no control over it, it just happens. We have tried to disconnect the system, but we weren’t able to, and the mechanic at the Toyota dealer said it can’t be done. He replaced a relay thinking it would fix the problem, but it hasn’t. Any suggestions??