It's so easy to lose track of a pet. But getting your pet microchipped is quick and pain-free; it takes your vet about 1 minute to insert the chip under the animal's skin (usually at the back of the neck) do and you 5 minutes to send in the registration. The actual chip is about the size of a grain of rice and contains a passive RFID tag.
And from then on your pet is quickly identifiable by any vet or animal shelter. I know. We had a stray cat turn up with us a couple of summers ago. We fed her and took her to the vet for a check-up. It took the vet 30 seconds to scan the microchip and then about 5 minutes to find the owner's details on the national register. The vet contacted the owner and there was one happy owner reunited with his cat who he thought had gone for ever.
There's more on microchipping at www.rspca.org.uk/allaboutanimals/pets/general/microchipping and Wikpedia.