Thanks everyone for their replies. I’ve clearly been too blasé about these things. I was reading this thread thinking overhead cabin theft is a thing? Really? Until it happened to a woman sitting a few rows ahead of me on an Emirates flight today.

Hilariously though, when attempting to demonstrate that she had not mearly misplaced the item (not exactly sure what it was) she opened her bag and accidentally revealed the business class blanket that she had stolen to the flight attendant.