Hi Paul,
Hmmm. Just taking a stab at it here. Are there any electrical switches near the seat, pedal, mowing deck or throttle area? I'm thinking it's an electrical sensor thats causing the light on the dash to go on. My Kubota has several sensors that detect if somebody is sitting in the seat for safety reasons (actually just an electrical plunger type switch). The switch might be broken or have corroded wiring connections.
I have a bugger of a time in the winter when these switches freeze solid due to ice and snow,,,sometimes it will shut down the entire tractor.
Regards
Jeff