It's rarely temptation, and almost always a simple error or laziness on someone's part. It's still a good suggestion to mark up returns though. I would add that including a date isn't a bad idea either.
I can't even describe how many times I've found cores in boxes for starters, alternators, etc. Usually because someone assumed the box on the counter that looked new was still a new part when in reality the fella who just sold it clocked out for lunch and didn't tag the darn thing.
I bet that one was a return and was going to be sold as used or some such but it either got entered into the system as new or a newbie yanked the used part to pack the box and not a new one. Most younger warehouse workers barely know what an engine looks like, let alone a particular brand of speed parts.