Although that sounds like the best explanation, my follower (Lydia) has lost 3 bows and 3 swords in the past week. And, we were not/didn't fight anyone that uses that particular shout. They have just disappeared. I replace them, and it happens again.
I have not been able to connect it to anything that we did in-game.
I think that's a different problem. I think that Lydia *does* mysteriously lose inventory items. I could swear that's happened to me several times, so I never give her anything really important to carry. It'd be nice to get an acknowledgement of the problem from Bethesda, but near as I can tell, they don't usually acknowledge a bug until it's part of a published patch.
At first, I wasn't really positive that Lydia had lost my stuff, but I've seen a number of other people reporting this, so I strongly believe this to be the case.