OK, I'll play.....could be a hidden toggle. I reckon if you leave the battery hooked-up, it will drain it sitting around (wonder it don't burn-up a coil too).
Anyway, battery power is fed through the main 20A fuse on a red wire (~14AWG) spliced into the battery positive cable, exits the fuse box also on red wire that goes to the key switch, then exits the key switch on a brown wire that goes back to the fuse box and feeds the remaining circuits. So, if you have power always, the key switch has failed in an unusual way or someone has (somewhere) jumped the red wire to the brown wire.(guess #1)
Such things can be tested for with a multimeter