No one in reception should have told her that "this never happens" - that is something that I hope Stephen (if he still is there) will pick up on and have some appropriate conversation. Even if you need to charge her for the lost towel, you do not insult her with such a statement.

Hopefully, this is just a one time aberration as people (including us) will leave towels and stuff on chairs all the time to take a walk, go in the water, or have lunch. Usually, in my experience, people do not specifically ask others to watch their stuff if all it is is a towel and some miscellaneous stuff of no particular value. Peopple do keep their eyes open generally for each other, I think. Some folks go out early and put their towels on their chairs and then go for a walk to the other end of the beach - never giving it a thought that someone is going to steal a towel.