Ours has a sensor beam near the bottom - if something is in the way of the beam the door goes back up. Sometimes something will fall over (like a rake) after we drive off and we come home and find the door open. :)

If you don't have a sensor, could it be an electrical short or bad connection?

Donna

A couple of reasons this may be happening, a circuit board could be going in your unit or someone may have the same code in the area. If this only happened once then it could have been a coincidental occurrence but if is continues it may be time to replace the unit.