Thieves can also open your garage door by just driving through the neighborhood and clicking a garage door opener. When they find one that opens they rob the house. Openers have codes that can be opened by other openers. It's just a matter of finding the right one.
Even though you live in a gated community the thieves could have climbed over a fence or entered the area by some other means than going past a guarded entrance. It's always smart to protect yourself against the possibility of a break-in.