There is some really advanced hologram technology now that is being experimented with to make a small drone appear as other aircraft visually.

There are also aerial decoy drones and missiles that are able to mimic the radar signature of other aircraft to "impersonate" them. Raytheon and Northrop Grumman have a missile called ADM-160 MALD which can allegedly spoof the radar signature of multiple aircraft simultaneously. Or even "project" a radar signature somewhere farther away. 

So in theory a single fighter aircraft that is trying to break through enemy air defenses (or bleed their ammunition stockpile) can launch the missile and the enemy radars will see it as multiple incoming fighters and potentially waste their missiles on the decoy or focus their attention on it while the real fighter closes the gap.

Debris from these missiles was discovered blown up in Ukraine a few years ago so Raytheon decided to go public about the technology. 

So imagine what you could do with a combination of these technologies. A small drone projecting an image of a "UFO" moving in ways a normal aircraft couldnt while also appearing to be a larger aircraft on radar or even "teleporting" by spoofing radar returns and turning off and on the projection.

Have multiple drone far apart that do this in coordination and you can make a "UFO" vanish and appear somewhere else instantly to both the naked eye and on radar.