No additional alarm, just the normal GM door opens while locked horn beeps, similar to the panic button pushed. i wonder if there is a central locking celiniod that might be going bad that won't stay open, basically telling the car it's still locked after it's been unlocked. Like I said messing with today's electronics is one of he toughest things (I work for the state as a mechanic/maintaince).I'm late to the party here so bear with me.....you mentioned the alarm went off.
Yes I know the car has a basic alarm but do you have an additional, dealer installed unit that uses a different key fob.
Dumb Q, but we/I gotta start somewhere.