My mother-in-law had a simple way to fix this. On the guide rails there were several square holes in the rail. Just above a wheel, next to a hole she would stick the handle of a small pan in the hole. There is no way anyone could open that garage door.
Did she park in the garage? If so, what'd she do when she left?
Nice trick after she pulled her car out to leave.............
Seriesly, I do similar when we are going to be away for a few days. Garage doors do open mysteriously from spurious signals. Was sort of hilarious in a new development we moved into where the moron builders had used the same code on several house garage doors.