I did rescue for many years and started chipping dogs in 2000. I gave the owners instructions on registering the chip.
A few years ago, one of my rehomed dogs was lost and the owner notified me, as required in my adoption contract. I called AVID. The owner had NEVER registered the microchip. AVID did allow me to give my information without charge, because the dog was lost at the time.
I suspect a LOT of people don't register the microchips when the dogs are chipped at a shelter. The microchips from a shelter trace back to that shelter, as we buy them by the boxes.
I guess if a person privately pays their vet for microchipping, they are laying out a nice chunk of change for the procedure, and may be more inclined to be sure to register it.
Edit: This doesn't sound right, in re-reading it. I'm not saying you didn't register the chip. It's possible AVID isn't showing up in whatever search blocks you are finding on line. I wouldn't get too excited until you talk to AVID on Monday. Your dog should have a tag with the 800 number on it for AVID.
There are several different registries. AKC has one, I think it's callED CAR - Companion Animal Recovery or something to that effect.
Also, a number of years ago, Banfield came out with their own microchips. There was some kind of legal issue with it because the Banfield (Petsmart) microchips did not read on the same frequency as the AVID and Home Again microchips. I did not follow that closely, but there must have been some kind of settlement, because we (shelters) got a free scanner that not only reads the Banfield chips, but Home Again and AVID.
It would be simpler if there was one microchip and one registry, but that would be too easy.