To be even more specific, the towels were in a drawer that is in a closet, so both were opened.

This is a hotel where I stay frequently and they do many extra things for me, so I didn't want to rock the boat. It's also a country where complaining is culturally frowned on. I keep everything important in the safe. One of the things they do is unpack for me when I get to the hotel around midnight after flying for 30+ hours, so they've seen my underwear already. My guess is he/she could see inside the drawer when the closet was opened to hang clean bathrobes. It didn't feel sinister - nothing has every disappeared from my rooms in dozens of stays and little staff turnover - just annoying.