Odd fault, any ideas?

[That Guy]

Hi guys, I'm not that clever with electrics so some pointers on where to start with this would be great: Whenever I leave the key in my M900, with the ignition key in the 'off' position, the battery runs flat, over say about 10 hours or so. I can't work out why. The lights aren't on, the ignition is not on and the key barrel otherwise (and rest of electrics) function perfectly. It is easy enough to take the key out to stop this but I was wondering if this indicated a short somewhere- and maybe a précis to a looming, more immobilizing, problem. Any thoughts welcome.

cheers
Guy