Man of Honour
Most chain gym codes are not user changeable, or at least not in my experience, they're also a lot longer than a PIN.That one sounds like the most likely theory to me - I bet loads of people use the same gym door code as their cards.
Just need to peek over their shoulder when they're entering it (or do a hi tech thermal imaging approach of taking a picture of the keypad, or some other method) then watch which locker they use. People are probably less protective / wary over the pin they use to get into a gym than when getting cash out, so probably easier to spy on them (even if it's the same number).