But of course, the Soviets were very interested (or the US believed they were very interested) in knowing about US military planes and their capabilities. There's an anecdote I recall reading - I apologize for not having the link handy - about how, at Area 51 (their airfield for testing experimental aircraft), they would leave big airplane-shaped objects on the tarmac to create cold spots for Soviets' spy satellites to photograph in infrared, corresponding to planes that didn't exist.
The reason I'm unable to find that anecdote is because the search term "Area 51", combined with any other search terms, brings up an enormous a flood of incoherent crackpot ramblings. I'm pretty sure this is the result of a deliberate strategy: create a bunch of fake information, so that real information about US military capabilities will be harder to find. A few hints and a few taunts, and voila, every discussion about unidentified aircraft is guaranteed to have a schizophrenic walk in and start rambling about aliens!