Question for dog trainers!!!?

I own a GSD, when his out in the garden and we are at home, he doesn't do anything silly and he behaves really well. when we go out and leave him free for a while even for 20 minutes he destroys the place i wouldnt leave out alone if i know i'll be long or none in. i tried to hide at home watch him behind the curtains so i can correct him , but he knows we are in so his an angel, he waits till we drive off or leave the house!!!