Black object with only one big white light on the back (no other aviation lights) moving very fast without sound in NW direction

Black object with only one big white light, not in front, but on the back (with no other aviation lights whatsoever), moving very fast, without any sound, in SE > NW direction in a clear twilight sky with no stars visible yet.
A first I thought it was an helicopter, because it was flying at what looked like an helicopter altitude, but because it made no sound, had no aviation lights, and moved much too fast for it to be an helicopter or even a jet fighter (in less than a minute it disappeared over the horizon) it occurred to me that it could be a UFO.