Hi,
As your dog is proving a little hard to catch in the act you could try going through all the usual actions before you go out. Put your shoes on, pick up the car keys etc. Give him ten minutes to get stuck in and sneak up on him.
I have to say that I think it is unlikely this will work, but it's worth a try.
My other suggestion is simple, Never, ever leave him in the yard on his own. Even if you can see him from a window, as soon as you move to a room or position where you can't see him call him in until you can watch the yard. This will eventually break the habit but you must be diligent and never lapse.
Sorry I can't be more helpful but unless there is a way to catch him at it I think this is the only safe option.