2001 Pontiac Grand Prix Horn

Every night (usually very late) for the last week or so, the horn goes off and the lights flash as if the factory theft deterrent system has been set off. I am able to turn it off using the key fob and verify the vehicle is not being messed with. I replaced the battery, but that did not solve the problem.

Please help, I'm sure my neighbors are ready to kill me.

Thanks in advance.