Come on if it's not space aliens what can it be? I'm not saying it's space aliens, I'm just saying, it's ****ing space aliens!
To be more serious on the topic, it could even be space aliens! I'm giving it a 0.4% bayesian prior chance (fancy name for "subjectively taken off my ass") of it being so, which is probably too high a percentage still, but hey! I can imagine a mega civilization with its own Prime Directive trying to subreptitiously observe us and once in a while make a stupid mistake and let themselves be slightly observed.
I must say, I have a harder time imagining these things landing and abducting people so they can have weird sexual fetiches with the subject's asses, but hey, I'm trying to be open minded here! Who really knows what the ****ing aliens like to do in their spare kinky time!