Anime: Koro no Dai Sanpo

Koro is a puppy owned by a little girl in a small town. One day she leaves the front gate ajar when she goes to school. Koro slips out and wanders the streets of the town. He is picked up by a girl riding a bicycle, but slips away at the level train crossing. He meets a friendly old dog and his kindly owners, while his mistress and her father ride around town putting up Lost signs on the power poles. Koro eventually falls asleep under a delivery van, whose driver gives him a ride. When he gets near home, Koro smells the familiar neighborhood and leaps out of the truck and into the arms of his delighted mistress.

Source: nausicaa.net/miyazaki/museum/koro/

Note: The second in a series of short films produced to screen exclusively at the Studio Ghibli Museum in Japan.
There has been no commercial release for this film, which premiered at the museum on 1 October 2001.
Note 2: The small town in real life is actually the area surrounding Studio Ghibli.