the identity of Nimrod
So in the episode "The Missing Klink", we learn that the Allies have a superspy who has been bedeviling the Nazis for some time, a mystery figure named Nimrod. We never learn Nimrod's identity, and he's never mentioned ever again as far as I know.
Now I don't imagine the showrunners gave this any thought beyond this episode, but if there was a Nimrod running around, my money's on General Burkhalter. From all appearances he was a perfectly loyal Nazi, yet he kept Klink in charge of Stalag 13 despite all his screwups (ostensibly because Klink was good at keeping prisoners from escaping), and as the guy who was in charge of Klink he could control how much anyone else knew about Klink's incompetence / the odd shenanigans at Stalag 13. Also, as a general, Burkhalter would be in a position to manage any number of operations or provide cover for them.
In the episode in question, General Burkhalter was one of the very few people who would have had access to the model dirigibles, if you saw the episode you know what I mean.