@Karen248, you could send a friendly note that says something like, "Could you let me know where you hung your towels? I know it sounds strange, but I cannot find them. Many thanks!"
I know. It still comes off as passive-aggressive. I had a steel bowl disappear, and worded my "can't find the bowl" message over and over in my head, but never did ask. Just wrote it off, as it was very inexpensive and definitely replaceable. And maybe they didn't take it and it will turn up someplace I never thought to look. Hopefully, if/when your towels do turn up, they are not in a mildewed heap under the bed...