Well, in the 21st century, I'm sure most grocery stores have computer software that tracks invoices and inventory levels by product. If they are re-ordering goods based on a visual sweep of the shelves, that's more like 3rd world problems.
Close. It's not invoices that are tracked, but scans through the register.
It's all based off of an items "Balance On Hand", so when you get a case of something, say it's packed 12, then 12 units are added to the items BOH. If 8 of them are scanned out the front end, the BOH is now at 4. Items also have a "reorder point", so once the inventory level on the shelf reaches a certain number of units, the system will generate an order for another case.
When you go to grocery store or convenience store and they are out of Product A that I want but have a surplus of Product B that I don't want, so to hide the hole, they put extra of Product B where Product A would be so, although it "looks" full, the flip side is that the product I actually want never gets ordered because nothing looks out of stock.
It depends what the store's policy is. Generally, if it's a hole, leave it a hole. But some stores may want to cover the holes with adjacent product for presentation reasons.
For stores that Hand Order, yes, you have to be careful to recognize what is blocked over and what is not.