[bena]

My intuition says counter-intelligence. The Allies were using intercepted communications, visual confirmation, captured sources, etc.

You can send fake reports if you think the other side is listening. You can move or mock material if you think the other side is watching. You can feed false information if you are captured.

[bombcar]

And this is why you may be better off solving the “serial number problem” not by going to entirely random ones but instead change to one that implies false data that you want the Enemy to find.