I believe you are right on the going back to the leash thing.
It took me months to get my husband to totally escape proof the yard so I could let him outside without a leash but I can see how the little monster has taken advantage of that and learned that I cannot run as far as him. Sometimes he will even follow me to the door after seeing then "bolt" back to the backyard so he can stay out and munch on things. Even sometimes his favourite treats won't bring him in.......sigh....back to the drawing board..