Could also be one of your battery cables (loose/corrosion). Check the cables at BOTH ends... not just at the battery. If you see a lot of green or white powder, you probably have a bad cable that's not getting a good connection.
A bad or loose connection would likely set that alarm off (it's just like you unhooked the battery then hooked it back up again... it triggers the alarm). As OldSchoolF22 said above, you'll have to find some way to disable the alarm now that it's been tripped (most alarms will shut-off the ignition). Find that override or valet button.
Good luck!