After getting lucky on a couple, I quickly learned how frustrating even the most obvious UPCs can be to find. I figure there are some calendar or regional differences. Likely things that end up on the SH are older to start with. I am not sure if the UPC differs if it came from a different producer or distribution center. I never had formal merchandising training to know how it works. Sometimes, it is a matter of the store mix. I remember a regional super center's Easter basket assortment. I was close to the UPCs, but the mix in my are and markets, which I checked as I was inside, not running around to the locations chasing after it, never had the precise one.
Other items I get lucky from recognizing those small imported brands that might be in the independent markets. I have a lot of those around me that I shop in sometimes anyway, depending on errands.
It can drive you crazy looking though. I don't try too hard. I get selectively lucky I think because I choose to scan certain categories selectively and recognize the likely retailers to have certain things in the mix if it is not store specific.