Missing Premium Items
It's a new feature in the game. Neighbors can steal premium content from other neighbors. I actually stole the sign from you. Not sure which neighbor stole your museum though. Thanks for the free sign...
Forum Discussion
There is a known glitch, where someone else gets logged into your town and for some reason those mean people sell of premium stuff or buy premium stuff with that person's donuts, as I have read several times in the bugs&issues section.
