I will say this. There is and will be time shaved off the clock as long as the system they have in place is used. It is in a sups best interest to shave time and most due.
Chester, I agree, that there are supvs out there that will alter timecards. I've heard of some, and I've seen them get demoted and others fired. I've been in operations, I've never seen a supv (or mgr) fired for not making their numbers. (They get harrassed etc, but not fired). However, I have seen them get fired for altering timecards which is a nice way of saying stealing from their hourlies. Some supvs may think it's in their best interest, but how can doing something that you can get fired for be in your best interest? If I were you, and knew of a supv that did this, (let me be clear, proof positive that they did this) I'd contact the corp compliance number. NO EXCUSE for this behavior.
If it was just a feeling, I'd check and double check to make sure what had occurred. Keep in mind, there's an audit trail, so you can see who changed a timecard. Some people think they are smart and use someone elses ID. If you see that Supv A ID was used, and you are pretty sure it was Supv B that did it. I'd t/w Supv A, and ask them about a time card change etc. Basically I'd get them to change their password.
Do the right thing for your people, and your people will do the right thing for you.