That has always been a gigantic fear of mine- I would be walking up to my bike, fail to notice the person sitting on it pressing down on the key, and they would do exactly as you say once I was w/in 5 feet or so of my bike! So what I do is to stealthily sneak up to my bike, hiding behind bushes, fuel pumps and any / all other obstacles that will conceal me. Sort of like Wile E. Coyote sneaking up on the Road Runner by swiftly springing from behind rock to behind rock until the Road Runner is w/ in the last leap. If I go to, say, a gas station with large, open areas between the parking lot and the exit of the building, I put up cardboard cutouts of boulders located strategically and then dart between them until I am close enough to my bike and the Road Runner er, potential bike thief to confront him or her directly.
I think this is a highly under reported and under funded problem on a national scale and hereby propose we create a group dedicated to the preservation of empty saddles on our bikes when we are not in attendance. Perhaps the old 'the tack under a piece of paper on the saddle' trick would be a righteous beginning?
Brian
Of course if someone wants your concours they just hop on while you are in the gas station with the bike in first gear, the clutch pulled in the stovepipe pushed down. As soon as you walk close enough they start it and are gone.