I'm not sure what the term management uses is----I hope Dragon reads this and helps me out----but I do know that once you punch out you are supposed to leave the building and not return to your work area.
Dave is actually right here. If a rank-and-file member is staying after work, off the clock, to observe supervisors working and they refuse to leave, they can be disciplined. However, trying to chase off a steward who is off the clock and investigating a grievance could lead to labor charges.
That being said.. if you're told to punch out and think a supervisor is finishing up, maybe you go take a dump and wash up every day after your sort after punching out. Maybe you're also a forgetful guy and always leave your work gloves, time card, lunch bag, water bottle, etc. by the time clock and have to rush back 15 minutes after you've punched out to grab it. We have a fast-paced job where it's easy to forget things sometimes.. and if you see a supervisor doing work you're qualified to perform, you're just being a good employee by offering to stick around and do it.
In order to get paid, you need to have pretty accurate times of when a supervisor starts/stop working and you'll also need witnesses. For example: "I saw Dip

McJackass (preload FT supervisor) begin loading package cars on 1/23/14 at 5:15 AM and he finished at 7:00 AM. Joe Union and John Union witnessed this." In order to make a point, I would always ask the supervisor performing bargaining unit work if they wanted me to do that instead. If they refuse, put in the grievance: "Grievant was available and qualified to perform such work but was directed to go home instead." Nail them every time for it -- even if you don't get paid (much), you'll make your point.