It was a dead bulb. Got a matching bulb from the local parts store and it works and the idiot light went out right away. I can't explain the earlier behavior - that's one for the Journal of Irreproducable Results.
2005 X3 3.0i, CPO, white, auto, adaptive xenon lights, stock suspension, premium pkg, cold weather pkg, heated steering wheel. Self installed: factory hitch, paint protection film, mud flaps