Lost. Money.

I've been paying good money to the local gym, but so far I've only been going there only once a week on average. Today, I decided to get some workout, and what have I to show for my troubles? An empty wallet. Only discovered the crime when I got back to the office. At first, I thought my colleagues were playing a prank on me, as I sometimes leave my wallet on the table. They all look innocent enough. Only other time the wallet is not on me is when I was at the gym earlier. Lucky for me, nothing else was taken.