Very small dogs (ankle-biters) roaming loose; is that a case of very smart dogs or inattentive owners?

Quite regular I see small ankle-biter dogs (those so small you can place them in and purses or punt them like footballs), running loose through a neighborhood. If they got into the street even if a driver was paying attention he/she would have a hard time not turning one of those ankle-biters into a throw rug if the dog darted in the path of the vehicle. There is no person observed walking them, and no one out in a yard that the dogs could belong to. It makes me think if it was a case where the dog(s) are smart enough to defeat whatever fencing that was erected to keep them in their yard, or inattentive owners who don’t have mesh, wire, or such to keep the small dogs from breaching the borders of the yard and getting loose, or that even if the owner is aware of the dog being in the yard they are not watching well enough to detect their pooch has found a way out?