If it's one of those old $hitty ones from the 70's with a master key my GF's condo one did that out of nowhere. I actually pried the master face open far enough to get a screwdriver in therea and punched out the piezo siren.
left it with a relay clicking, then that went away after a day.
If it's a later one with battery backup find the AC source, unplug it then find the brain pop it open and yank the battery backup.
Time to be a butcher
