About 20 years ago, when we didn't renew our dog's license with the city, someone came knocking on the door when no one was home to answer except for the dog himself.
They had proceeded to slip into our mail a card warning us to renew the license within x period of time, or else.
I doubt they came due to neighbourhood complaints though he used to do his howl whenever the last person has left the house. This generally lasted for the first 5-10 minutes.
About 5-8 years ago (can't remember exactly), someone had gone by my then boyfriend's family's place. His sister answered the door. He asked specifically for our dog who was originally registered to that address when we first got her and got a license from the city for a couple of years. After having not renewed for a couple of years after the last, I guess that's when they decided to make their rounds. I'm not sure if any attempts were made previously, but of course, there was never a dog around to "answer the door" when we weren't around because we never left her there when we weren't. This time around it definitely was NOT due to any sort of complaints. Our girl doesn't bark and scream and cry when left alone and there isn't a stinky yard for any one to complain about smells. We walked her and picked up after her.
His sister said quite honestly to the person, "[insert pet name here] is no longer with us." That is the truth no matter what it had implied
since we hadn't been staying there for at least 2 years at the time.
So it's not always for "animal control" or for "checking on humane living conditions"... in the end, a lot of times, it's all about the $_$.