Wot the others said.
I worked two yearsish in big store, lots of operations and replen ( stock) as well as tills so in all roles really.
Yes the MANAGERS frequently had to come in unexpectedly for extra hours eg if there was a serious theft or meeting ( the sight of our deputy in his hoodie and "home gear" was quite common)
Also sometimes us hourly lot were "encouraged in strong language"
to take on overtime.
And yes sometimes an extra half hour might have to be done (but we'd take it back later) Or sometimes the managers would underpay everyone's overtime ( probably fiddling the monthly figures).
But everyone, from the sixteen year old temps to little old ladies working to supplement their pension would chase management and get it back.
It was tough and had a strong "edge" and dog eat dog atmosphere but we definitely weren't unpaid slaves for whole days!
Oh, there was one poster up once suggesting any "volunteers" could come in unpaid to help clean the store offices for two hours, which was completely ignored I believe 
SOMETHING is very off here. I agree the "someone he fancies there" could be a possibility. Because it's such an intense, stressful environment a lot of liaisons start, and not all of them seem to be singles getting together.
I am very "average" looking and not very sociable/ flirtatious and quite a few of the partnered men ( at all levels) were "scoping me out" and throwing bait down to see if there was anything going on - and would've covered for each other as well. So I'm not sure how you can find out. Best of luck op 