I happened to be in the bottom of Deceit during a respawn earlier today, when something spawned on me that I thought was a slime, but what later turned out to be a doppleganger. Me and my dragon took it down pretty fast, so it didn't have time to change shape.
I then went over to check the loot on it, and was quite surprised to find a deer mask and a druid staff on it. I thought that the masks can only be found at the AG. My first thought was that someone had died there and lost their staff and mask, but the items were not .owned, and the druid staff was brown (looked like a gnarled), not green like they usually are. Besides, I had just seen the creature spawn, so there's no way it could have looted anyone.
So I'm pretty sure this is a bug, resulting propably from me killing the dopple before it could change shape. Or are dopples meant to drop masks? And if it's a bug, should I just trash the mask?