Back in the early days of TCAS, I was reviewing pilot squawks that had resulted in the TCAS processor being removed. There were a few strange writeups, centered around southern Nevada, such as, "We had four targets, our FL350 altitude, approaching from four directions. ATC said no other a/c in the area."
I called a meeting of the airline's engineering and Flt Ops, some of which were USAF Reserve pilots. I explained this was surely the work of ECM, Electronic CounterMeasures at Nellis AFB, (or even Area 51?). I suggested they tell the USAF to knock that !!!!! off. We never saw another such squawk.
GB