If a towel is left on a fitness machine at a gym, more than likely it was forgotten there by its owner. Even if it is obvious one is attempting to reserve the machine, I think the policy at most clubs is essentially, "Move your feet, lose your seat." You don't own it, your name's not on it, it's fair game. I don't believe it's fair to make others wait while someone goes to the restroom, others who perhaps took care of all their business before venturing out onto the gym floor. I suggest you visit the bathroom upon arrival, then choose a machine.