joining any dots?
If 2 plus 2 always made 4 we'd have the answers to the world's problems. Seeing as you're not going to accuse her to her face, and don't have any proof, just write it off?
Go out and buy a replica - leave it next to her again, and wait and see what happens.
If the second one disappears too, then you a have a serial scarf thief. A little trial and error, or establishing a controlled experiment, doesn't help at all, but might make you feel better.