They do have a wide circling area and they also seem to suffer ADD. They get distracted very easily, I've seen them head away from fighting guards to go after a deadly chicken.
The Dragon may have headed off after another enemy and they will wander far. edit: The Dragon may have also known found anything el;se in it's agro range and settled down.
As for markers, some Dragons will hang around specific locations. These show up as a Dragons head on the map.