So what was it then? If you say (in the O.P.) you are less then 20' away from your drone, and how this "thing" moved past (towards you?), it must have almost hit you in the head.
Edit: No worries I've figured it out after a few thorough re-views.
It's the way you initially look at this video, if we assume "something takes off and fly towards your drone" its so spooky and UFO-ish.
But look at it again, watch the lights on the horizon on the left. YOU are descending, and then going fwd past a fixed object, traffic sign or something taller.
If we look at it that way, the "object" is not moving any longer, but YOU are.
When the object is on top of the screen right, it blocks the lights on the horizon under it so you can see it's mounted on top of a post or pole.
You were very close and lucky not to hit it.