The OP seemed more intent on getting the actual thermos back (the remarks about lost & found) than in the act of something going missing. I would say keep a better eye on your stuff in the future, and my money is on that it was given to the wrong person. If I got home and discovered a strange thermos in my bag, I would likely just toss it.

Unless someone had filled it with diamonds. Then I would have a great moral dilemma on what to do.

Cali