[Jolien]

Change into your gym outfit and go commando and change your normal bra for a sports bra.
"Forget" to put the bra and panties in your locker and leave them out in the open
If they are gone when you come back, strip naked and go around the room looking for them. Asking people if they seen them.

If this is not daring enough do it in the men locker room

I can do this either at the gym or the pool