Independent of the type of ignition, as reported earlier in the post, if you have an electric fan on the radiator or an
oil cooler, and it is hooked on the same circuit as the ignition, after you turn off the key, the fan will act as a generator and provide enough power for ignition (and ECU if you're using one) for several seconds (seen as much as 5 seconds) after the key is turned off. The engine keeps on running like you never turned the key off. I've seen this happen on several different vehicles. Unnerving at first till you understand whats going on.
Dave