If this dog is allowed a free run of the garden when it is not being supervised and it knows you are not there, I would question how secure your garden is. My cross is the biggest dog I have (he is about 28in to the shoulder and weighs about 35kg). He can Squeeze through tiny gaps, and 6/7ft fences will not keep him in if he wants to go.
He never wants to leave my area, but will do anything he can to get to me.
I'm sure he sees each obstacle as a new challenge