Police said Charles McCowan parked his pickup in front of a mini-mart in Azuza, California, last Wednesday, leaving his Boxer named Max in the passenger seat.
When officers arrived, they found the pickup across the street in a fast-food parking lot but had no idea how it got there.
In security video shown Thursday on KCAL-TV, the truck can be seen rolling backward out of the store lot and across the street, threading its way through traffic and out of view.
Police said that after McCowan left the truck, Max knocked the vehicle out of gear and sent it rolling backward.
Both Max and the truck emerged without a scratch.