Unless your gas line is fed from the front side of the house to the meter. Then you would have squat for markings if you only requested the back yard. They will only locate what is requested, to the letter on your description.
Was a sketch or clear ticket left ? Or if a dog is in the back yard the locators will not even attempt to enter the back yard.
I once saw a Ditch Witch hit a 2" IP gas line, damn awesome sight.

