I like science fiction, but the first thing that comes to mind isn't aliens, but spoofing. Suppose you had a drone with radio gear aboard that rebroadcast the radar signal with a slight delay. It would appear to the radar to be farther away than it really was, and its speed (and acceleration) would appear proportionately greater too. Your eyes can see it, but it isn't easy to judge distances of strange objects.
That would be rather worrisome. If hostile drones can spoof their locations, our warships are more vulnerable than we think. And so, of course, are land installations--though land installations could more easily use info from multiple locations and triangulate the real position.
I imagine you could do some cute things with retransmitting a "locked-on" radar signal to try confuse a missile. You couldn't confuse it as to direction--the signal would stil be coming from your drone--but if the missile has the distance wrong it could detonate too late.