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A:
It is possible if you have very simple coding methods with your remote handsets, where someone could possibly find your code and be able to operate the door with a remote handset matching your operator make and transmission frequency.
Nearly all high quality modern electric operators use encrypted coding in the transmitter and receiver, which provides almost zero chance of anyone copying your remote handset.
If you ever lose your remote handset then be sure to change the remaining handset codes as fast as possible of course.