Happens all the time in my house, but it's Haunted.
My GrandMother passed away in the house, and since she died, things go missing all the time. So if we notice something gone we say "Grandma, put it back." and usually within an hour, it shows back up, usually in a very obvious spot, and it's some place we've already looked.
For you, have you checked behind the sofa, or under the cushions, I'm talking behind the back of the cushions, like where the sofa back forms, it might have slipped into the frame, especially if she was aitting there.