Say you're running a black development operation. Maybe making some kind of stealthy recon drone or multipurpose recon/attack system. But this is very, very close-hold. You know politicians and their staffers might as well be a direct line to our adversaries. Regrettably even some personnel high up in the military chain of command have become more political animals than fighting men and women. So this is strictly "need to know" and no-one not directly involved needs to know. No one.
How do you test it? The contractor developing it claims it is of course the best thing since sliced bread. It is the be-all end-all of stealth tech. "No one can see this xyz high-zoot, velco-enabled, dolphin-friendly wedge..." You don't want to run big test campaigns. Aviation buffs and our adversaries watch for those, they are very public. But you want to put it through it's paces and verify that it can do what is claimed and that it can do it without getting caught doing it... So why not put it up against the best military tech out there - the US? Just make sure it is not doing anything overtly hostile that might get it shot-down / destroyed in case it does get detected. But make it an unannounced test. Good training for the military, valid test for the contractor's widget...
Of course not everything works quite as well as advertised on the first try. Some things might get detected due to unforeseen circumstances or plain old screw-ups. (defense contractors are human too) Deny you're doing anything - remember, no-one needs to know. Since these things literally are "unidentified" flying objects, lump reports into the files with the rest of the alien sightings and cattle mutilation crowd. Instant cover for your black program. Berate the contractor, come back with a dot rev, lather, rinse, repeat...
Modern technology has advanced so quickly that current concepts of warfare are not only outdated but are being stubbornly held by todays poliitcal, pole climbing, little real combat experienced “battleship” senior officers. Many brave young men will die on surface naval combatants and in massed armored formations, never seeing or being given the opportunity to respond to those who killed them.The cost of empowering the hidebound, politically correct will be terrible and steep.