Ok, so on today's flight the lady sitting two seats over from me (she at the window and me on the aisle) took my water.

Prior to boarding this morning I bought a bottle of water to take on the plane. I wasn't thirsty immediately so I placed it on the middle seat. When my row mate shows up I moved it from the middle seat cushion to the middle seat pocket. The middle seat remained empty. I then proceed to nod off for about 30 minutes after we take off. I wake up and look for my water bottle and find it missing. I see my seat mate drinking from a bottle like the one I had placed in the pocket and inquire if she had taken my water. Of course she immediately admits to taking it thinking somebody has conveniently left it there for her. She apologized and gave me the $3 I had paid.

So:
(1) Who would take a water bottle from a seat-back pocket without knowing the provenance of said bottle?
(2) Who, given the severe lack of freebies on US airlines, would assume that the bottle was placed there for her convenience? This was on AA and I assume this person had either sufficient status to grab the exit window early or travel savvy to have secured said seat by other means.

My solution was to give her what my sons have come to understand as a stern look with no further repercussions. I mean I wasn't mean to her afterwards or anything. But to somebody who knows me would have been able to tell I was not happy. I think she was sufficiently embarrassed to not finish the bottle once I called her on it. She may have thought I has already opened the bottle and drunk from it, but it was an unopened bottle - or had been when I placed it in the pocket.

To some degree, maybe it was an honest mistake, albeit difficult for me to believe. Anybody else have to deal with such transgressions from row mates? How did you respond? What are the acceptable limits for stern looks and stink eyes?