This was a claim I remember from my High School Aviation class, and possibly a few other places. The idea is that other pilots might see a propeller-less airplane being flown by a gorilla. They wouldn't tell anybody, because everyone would think they're crazy.
This seems like a myth. Fighter cockpits are cramped places as they are without wearing a gorilla suit, and you'd have to be flying pretty close to see what the pilot looks like.
I'm on my phone, but I do know this one.
This happened with the first US Jet (Bell P-59 Airacomet). In 1942; the airbase was Muroc Army Air Field (today Edwards Air Force Base). The Airacomet was such a secret that on the ground it was fitted with fake propellers to disguise the new technology of 'Propellorless aircraft', or simply jet technology.
There is a tad bit of conjecture here though - the new test pilots of the P-59 were drinking in a local bar with some P-38 pilots from a nearby airbase. Over drinks, they'd (drunkenly) bragged of flying this new type of aircraft and were branded liars by the P-38 pilots.
Now, the air above Muroc was restricted airspace - even for USAAF personnel. The P-38 pilots had been spotted trying to get a closer look over the airbase to substantiate these claims. As such, Jack Woolams, one of the test pilots, waited for one P-38 to fly past, take off in his Airacomet and catch up with the P-38. This part is a truth, it's the small gorilla mask detail that creates the myth. Jack put on a Bell Derby cap (a signature of those working on the Airacomet), supposedly a gorilla mask and an unlit cigar, pulled up alongside the P-38 and then waved as the opposite pilot looked dumbstruck.
The next day at the bar, they asked the pilots if they'd heard about one of their test aircraft missing and a local gorilla escaping from the zoo nearby. As the story goes, the P-38 pilots had proof the P-59 pilots were honest.
That is the story - Jack was known for his larger than life personality (as you needed to be to be a test pilot), and the details of the fake propellor, the interception have valid evidence behind them, but the gorilla mask itself? That one is up to the reader, and I for one always liked this myth and hope it did happen!