I've always operated on the theory that thieves are first lazy and then sneaky. A hidden switch works wonders. I personally carry keys for JCB, Caterpillar, John Deere, Case, equipment on my key ring. Those just about cover all there is there most days. I'm not the only one. I assume the thieves do too.
That's where a hidden switch or something really simple that cripples the tractor works wonders. When I have to leave my tractor at a construction site I use a Kryptonite cable to lock up the safety bar. That means they can't move it without cutting the cable. I also throw the rocker switches for the lights and emergency brake. That takes some of the silent and sneak out of moving it.
But a simple hidden toggle switch to the starter circuit behind, say a locked hood, or inside a locked tool box means they have to take the time to hunt and remove locks. They're lazy. Chances are they are gonna go on down the street for something not quite so contrary.