I was busy enchanting away for about 20 minutes in my riften house basemant and when I exited the enchanting window someone was standing right next to me!
Literally made me jump! I looked around and realized one of the mannequins had Walter over and was looking over my shoulder while I was powering up enchanting.
I went over to his vacant stand and I could activate Jim from the vacant spot.
Next time I came back to the basemant all was normal.

I have had this very same problem. Has anyone lost the armor they have stored there due to this or is this just cosmetic?

I had this happen in Markarth. I thought it was rather strange that there was a mannequin at the corner of the table, but eventually decided it was neat to have a "server." After a while the fellow wandered back to his base.

It's even more startling when you use a mod which adds a basemant room or so for smithing - and which comes with a mannequin that moves around a LOT.... and breathes.... I use as well the mod which makes the mannequins fem instead of male, and now when they take off their clothes and move/breathe, they're nvde. It's like a brothel sometimes.... Pretty funny!

Yeah, I had a Honeyside Brothel last night armor glitched and didnt show up my wife was like umm whats going on in your house? I told her just a little party!

I've had them relocated at Honeyside, but has anyone one witnessed them in motion?

Nope, it's always one of those horror movie moments. One minute they're where they belong. The next, THEY'RE RIGHT BEHIND YOU.

Easy way to reset them is remove/replace a piece of gear on them and they snap right back.