It is most unlikely that it was a former pet. A very very tiny percentage of people keep turtles as pets. In most areas of the country, turtles live all over the place, on land and in water. You might as well find a pigeon eating in the yard and guess that it was a former pet.
In addition, if the species is appropriate to the area, even a former pet (in that .0000000000000000000000000001%) will do fine in the wild. Turtles of the north american species seem to have no problem with release (as is evidenced by the success of released species in inappropriate areas).
Most likely it's just a wild turtle you are lucky enough to see, doing what it thinks it needs to do.