It is very irritating to walk into a store with a specific item in mind, not find it on the shelf and ask someone if they potentially have it in stock ("Excuse me, I couldn't find any Wibbly Wobblies in in the Timey Wimey aisle, do you have any in stock?")..........and the person goes and examines the same shelves in the same aisle I just scoured a minute ago, huffs and says no, they don't have it.
We get a variation on this at my library, and it drives me up the wall. We put patron holds out on shelves and put slips with the patron's name in each item. They're all shelved alphabetically by last name, so you just go to the appropriate spot on the well-marked shelves and take your items.
It is extremely common for a patron to come up to me and say they checked the shelf but their holds are not on it. When this happens, my first step is always to go to the shelf and look for myself. Sometimes the hold isn't there at all, and I have to figure out where it actually is. Sometimes the hold is just slightly mis-shelved and easy to miss but also easy to find if you're used to the system. At least half the time the hold is precisely where it's supposed to be. The reasons patrons don't notice that their holds are exactly where they're supposed to be include but are not limited to
1) It's a small item that got lost between larger items. Happens a lot with CDs wedged between books, for example. Perfectly understandable.
2) They were looking in the wrong section entirely. We have the shelves marked off by signs with letters of the alphabet on them. The number of times I have seen someone walk over to, say, the C section and carefully peruse all the last names starting with C before coming over to tell me that their hold is not on the shelf and that their last name starts with, say, R make me weep for the fate of humanity. And no, their first name doesn't begin with C either.
3) They didn't look through the entire section. We have one guy whose last name begins with Er who has on multiple occasions told me his hold isn't on the shelf. When I walk over there with him and find it exactly where it's supposed to be, on the second shelf of Es, he tells me he didn't realize there was more than one shelf. He apparently thought the E section ended with Edwards and didn't look at the shelf below that. I'm not sure what he thought was up with that row of books that separated the shelf he looked at from the beginning of the Fs. This happens a lot, though this one gentleman is the only repeat offender.
Now, I agree that grocery items on the shelf are not the same thing, but I would bet that the employees who check the shelf frequently find the supposedly sold out items when dealing with customers other than you.