Lost dogs, collars and micro-chips

Several years ago, my husband and I found a wonderful dog in a “kill shelter” up in Virginia. After he cleaned up and was behaving properly, we took him to our vet. The first thing the vet did was take a scanner and scanned our “new” dog for a micro-chip to ascertain if he had belonged to someone who had loved him. No chip was found. He did not have one.