Pre-Desert Storm the Army had some Soviet helicopters that they... "acquired". Some of the acquisitions were classified as were the existence of the aircraft. They would be quickly pushed in and out of a remote Test Activity hanger. If you asked you were told the MI-24 was a modified S-76.

After Desert Storm the Army made a big deal about getting ahold of some Iraqi helicopters and the MI-24 was suddenly painted in Iraqi colors.
I've also told the story of seeing weird looking civilian aircraft with strange antennas taxi up to an FBO. A bunch of military dudes trying not to look like military dudes get out- you know, short haircuts, bug out bags, sunglasses, polo shirts, khakis and boots. You ask about the airplane and instead of coming up with something like "We do aerial surveys for oil companies" they respond "We can't talk about it." Gets you curious. Let's check the N number. Oh what a shocker. It's owned by a CIA shell company.