I haven't opened my trunk on the Camaro yet, but I'd bet it's the remote in pocket. For my DD's I've set off the alarm multiple times, opened remote hatch, and had the remote starter start the car.
I thought someone was stealing my gas before I figured out I was starting the SUV accidentally. Lucky for us it's the car that's in the driveway and not the garage! I've got to the point I only keep my keys in my pocket when I have to.