The last litter of puppies I fostered, one of the pups was adopted by my housemates grandson, I didn't see the pup for a few years, but she remembered me. I think it is the smell. I was part of her world since the very beginning.

it depends on all those facr=tors - And that the odor of both is important. James thurber wrote a story about a mother dog and her daugteh being reunited wherein the daughter recognized her mother, but the mother did not recognize the daughter. Being James Thurber, he described it in a fancifu; conversation in which Mom had the last word: "I'm not your mother. I'm a motorman's glove."

I you frequent The Dodo on YouTube, you wil find many videos showing dogs fostered together reunite with their mother and siblings, and they all sseem to recognize each other. I don't think you'll find any of cats doing the same, though.