there are exceptions to every rule.
Just as there are excellent owners who don't have a fence - but use a leash when the dog(s) are outside - there are also dogs whose backyard is safely fenced, yet 'escape' when the front door is opened.
The point made about trying to call off a RR who is in high gear prey drive is true even of a well-trained obedience dog. Watch a coursing dog on the side lines and you will see that prey-drive trying to be controlled.

Let's face it, there are dogs who can easily clear a 5 ft fence and they aren't agility trained dogs. I have seen one RR scale a 6 foot kennel fence...and he didn't jump...he climbed it!

To date only one of my RRs has shown a desire to go over a fence. She was one of my puppies who came back for boarding. She lived on 75 acres near Traverse City and pretty much had the run of 5 fully fenced acres. She had learned to 'jump' the fence when the owners got a covered hot tub...and placed it next to the fenced area in the corner of the yard. She would pop up on the cover of the hot tub and bask in the heat...and then one day she put her front paws on the fence and over she went. When she came for boarding the owner mentioned that as he got in his car.

We put a solid tarp over the top and lobster claw clipped it snugly over lapping the kennel run. She tried several times to go over it. That made my decision to put her on a 16 foot flexi for yard potty time. Whenever she comes for boarding...we add the kennel fence topper.
To begin our training we do not allow any of our puppies to stand on hind legs and put their front paws on us, the counter, fences, gates, etc. When a dog does this they are gauging the height of an obstacle. We discourage neighbors and friends from allowing or encouraging the dogs to come up so they can pet them over the fence. I think the idea of adding a hot wire to a short fence might be a deterant for dogs who are already exhibiting the desire to jump.
Even where we live, the law requires dogs to be fenced or on leash. The only exception is bird hunting or rabbit hunting seasons.
Sandra