Our garage door at home open by itself from time to time?

Our garage door twice already has opened by itself, once when we were out of town and again yesterday when I got home from work it was open. The first time I erased all of the codes and reprogrammed the remotes and also changed the pass code on the outside keypad to get in. What can be the problem? I was told that the neighbors could be sharing frequencies with our opener but if that is the case how can I keep this from happening again?