Something Creepy at the Haunted Hangout

Today I was showing a visitor my miniatures. When I looked into the living room of the Haunted Hangout, I spotted a mini visitor. I've never seen this little sweetheart before. I have no idea where she came from. I phoned two friends and my sister, who I thought might have planted her there. They swear innocence.

I'm stumped. I can't think of any other repeat visitors who might be likely culprits.