Only ever as a joke with coworkers that I talk to when I'm doing my grocery shopping. I'll say something like, "Hello, I have a list of 100 items that I would like to individually ask you to check in the back for."
That being said, I don't have much of an issue with customers asking as long as they aren't rude about it, and I'm not in a rush, as while I'm part of the produce department, I also take care of the bulk department, and I'm on the only one who gets scheduled to do that, so if I can't finish that by the end of a shift, it's not going to get done again until my next shift. And on the one hand, that's on management for not giving me more hours, but on the other hand, I'm still the one that has to deal with it come my next shift.
As for actually checking in the back, I definitely do, and do my best to be thorough. In my experience, most of my coworkers do legitimately look as well, but how thorough they are varies. The problem is that we get...I want to say a dozen or more pallets in for produce each day, on top of what is still left from previous days. And some of that is stuff that doesn't need to be filled until later as it's still full from the previous day. So if an item is something we just get a single box of, it could be in the middle of any number of pallets that we haven't taken apart yet because everything visible is stuff we haven't needed to put out yet. My general strategy if I've been asked to check for something is to do a quick look over everything visible, and if I don't see it, then I'll check the order to see if it says we got any in that day. If we didn't, I'll assume we probably don't have any left over either (or else it probably would have gone out the previous day), but if we did get some, I'll have to start digging through pallet after pallet until I find it, so I understand why some people just won't bother if they can't find it quickly and easily.