Buick Anti Theft Activating ?

May sound silly, but how old is the garage? Potenial of a mouse, chipmonk or other rodent there?

Proably a good guess!

Remotely possible, but very unlikely

See if there is a way to decrease the sensitivity of the motion detector. On my Saab, there is a switch to do just that. That way when you park at airports, and other high-vibration/high-noise areas, the alarm is not always going off!

This is usually caused by a noisy door or lock switch. As temperature changes a noisy switch can make/break contact and set off the alarm. The hood switch is the culprit more times than not. Take it in and have them check it out. You will need a wiring diagram to do anything yourself. I don't know if that system has a vibration sensor but that could be a problem id there is anything causing vibration or shock waves. Also as mentioned a rodent chewing on the wires can be the problem. Good luck in finding it.

Did someone hit the panic button on the remote? Happens more often that you think if keys are kept in your pocket or the kids play with them. If I was to stoop down with the fob in my pocket, I'd often pop the trunk open. Now I put my keys on a shelf when I come in the house.

As for the dealer, he may never find the problem since it is not happening when he has the car. Unless it happens again, I'd not waste my time calling.

Happens more often that you

I have done this a bunch of times.

I have a remote starter in most of my vehicles. I've actually managed to start them doing just this!