Ghost story - the phantom GP20

A DC ghost story - and this has happened SEVERAL times. I have a Tyco GP20 #912 assigned to MOW service. The motor is absolutely silent - you cannot hear anything even with your ear up close. Now the layout is conventional DC block control. 912 will be resting somewhere unnoticed. I will be working switching cars somewhere, and lo and behold, here comes a headlight and 912 creeps into the block. Unsettling! Guess I have to watch those block switches a little more closely.