So those few dark bits that I thought might be termite droppings, alas, were. I pulled out the channel in question, and there were tunnels running the length of one of the boards. I'd already replaced the bottom plate, so I turned my attention upward, and removed the top plate and the doubler. To be sure, I even removed a ceiling joist and part of one of the roof rafters. At that point there wasn't much to hold the siding in place, so I removed it, too (carefully stacking it in a corner). Today after a dump run, I'll go ahead and remove the last vestiges of that wall and rebuild it. If I'd known, I would have just demolished the whole wall from the start and rebuilt it then. Sigh.
Sawzall Hero II