While I'm riding I leave mine hidden in the car and leave the car open. I figure by the time they've closed up the trailer ramps and found the keys someone will have spotted them as I usually park up next to friends and only really go to small shows where I know most people.
If I'm having a wander, obviously I take them with me and do NOT leave them on the bonnet then spend an hour in a panic thinking I've dropped them frantically searching every bit of grass I've walked on until a friend spots them....not at all.....nope, never done that