I can only assume what I was talking to was either an SR-71 or something being tested.
A lot of times, when deployed to Mexico or Central America, when I got bored at night, I would hang out with the Tower at MacDill AFB (on the radio at 8.993). We would BS all night and when they wanted to take breaks, I would take care of their radio checks. I had just done a radio check with "Air Guard 2905" (no idea what kind of plane it was), then I get an RC Request from "Classified". He was 5x5. He said he was at angels 30 (which meant he was at 30,000 feet), about 1-2 seconds later he wanted a new Radio Check from angel's 10 (10,000 feet). No call sign, and dropped 20,000 feet in a couple of seconds. Not sure what kind of aircraft that was. That was during the early to mid 1980s. Stealth was still secret, but being developed (not public till 1988). I just guessed SR-71.
Mike