Who Steals Car Keys??

We are members of our local YMCA and stop in most afternoons after work to do our half-hour, and I have a scantag on my car key ring that I use to get in. I always have to change into my workout clothes in the ladies restroom (not the most conducive place for changing clothes but the only viable option here), and ordinarily the first thing I do is to drop the keys I'm still holding into my gym bag. Today I placed them on the toilet tissue dispenser instead.
After our thirty minutes, as my husband and I were placing our gear in the back seat, I dug into my bag to retrieve my keys. No keys. No problem, or so I thought. I went back inside, fully expecting them to be at the front desk. They were not there. I checked the bathroom, the exercise room where I placed my bag while I exercised, and then dumped my bag on the back seat. No keys. Apparently whoever found my keys in the bathroom thought they had scored a big one and decided to keep them.
Who does that! If I had not been with my husband, I would have been so stranded. I cannot imagine an adult NOT turning in car keys, and I can't imagine why a child would want them. This one has me stumped.