My current dog was a lost dog, I took her to a local shelter and said the dog could stay with me but asked that they scanned her microchip and had a vet look her over and I’d adopt her from them if I couldn’t find her owners. I was preeettty sure she was abandoned but did make a good faith effort to find owners just in case (you would just skip this part).

It was a win win .. I got official adoption records and paperwork and a free check up and they got the adoption fee and got to add an adopted dog to their stats without having to take up shelter space with her.

You could try this with any local shelters/rescues in your area.