There is also one strange, remote possibility, but worth mentioning. Do you have an electric fan? If so, they will generate voltage until they stop spinning. My 27's electric fuel pump and headlights will stay on after I turn off the key for a few seconds and I found out if I shut the electric fan off first those die immediately. A fan motor can become a very efficient generator.

If yours is wired up so that juice can backflow to the coil from the fan, that might be your cause. If you have an electric fan, controlled by an on off switch, do what I did and shut the fan off first and see what happens.

Don