I've seen one remotely plausible idea for these things
The concept of a large, experimental military lighter-than-air vehicle, designed to be invisible to radar, and with a lower surface capable of imitating the dark, star-strewn sky above it. Such a vehicle could seem to "vanish" if it activated the camouflage effect.
However, the behavior as described, running around over inhabited areas, flashing bright lights, makes no sense at all in this context.
In fact, one of the main problems I have with most UFO sightings is that they make no sense if one assumes they are operated by intelligence (local or otherwise) that is trying to avoid discovery.
Believe me, if we were really being observed by aliens, you wouldn't see them.