oh lord, I wouldn't dream of trying without a leash! I have a hard enough time keeping him from escaping when I'm going in and out of the apartment. He absolutely does not believe me when I tell him there's nothing he wants out there (we've got below 0 temps right now, and he'd get lost in the snow drifts) but he doesn't believe me. fortunately, the few times he's made it past me, he only goes across the hall to scratch at the neighbor's door mat.
I'll be very cautious in trying this. he's a bit of a brat, but I'm not willing to lose him.