This can arise through an unnoticed programming error. Suppose you want to program 4-5-1-2. But when the unit is in program mode, you forget to put in the two digit memory slot prefix but just directly enter 4-5-1-2. The microprocessor will accept this sequence and interpret it as code 1-2 in memory slot 45. You test the code by reentering 4-5-1-2 and the door opens because the sequence includes the real code, 1-2. Later it will be noticed that the door is opening on 1-2 as this is such a simple sequence that it will be entered. When you have any evidence that unknown codes (particularly short ones) are in the unit, it is best to erase all User codes and reprogram.