Indeed, it was the glovebox light, intermittently not turning off. When I checked it before (two weeks ago, not a week - time flies), the switch was working; not this morning. Time for a shim on the door.


At the moment, the solution was to remove the bulb.

For what it's worth for other folks tracing this kind of problem, I pulled fuses in groups of five to isolate the group in which the drain was occurring, then replaced them one by one in the problem group to identify the specific set of circuits involved. With the huge number of fuses in the 850 (compared to my 1949 Studebaker truck, which had exactly one), pulling them one by one would have taken a long time.