"I've seen instances on various brands of cars where an operating heater blower or radiator fan acts as a generator when the key is turned off."

A permanent magnet fan doesn't de-energize in a surge as pure inductor would, because the momentum of the blade keeps it energized. What happens is that when power is removed, it instantly becomes a simple DC generator. At first it will produce 14V, with the same polarity as the original source. As blade momentum is lost to friction, air resistance, or back EMF (if the connection is complete), voltage gradually drops to zero. However, in Mercedes systems, fan power is supplied directly from the battery. So when the switch is open, the fan's generated output would be on the other side of the ignition switch from the indicator light. The current it generates simply charges the battery until it stops.

I think what you're seeing a reverse surge from the alternator winding, powerful enough to jump the gap in the ignition switch. My guess would be a weak trio diode, and maybe carbon tracking in the switch. You might be able to confirm by comparing the ripple currents on B+ and D+. If it's not that, then somewhere a shunt resistor in a relay has gone bad, and it would be like finding a needle in a haystack. Not sure I'd bother fixing this unless it became a real problem.