I started getting an intermittent rubbing sound. Applying the front brake would stop it for a while. It got worse as if debris were caught in the caliper. I removed the wheel and cleaned the rotor and the slot in the caliper with a paper towel. No change. The clearance looked good, but I realigned the caliper by loosening the bolts and lashing the lever to the handlebar. No change. I removed the caliper to inspect and clean the pads. With the caliper out of the way, there was no rubbing noise.
When everything was cleaned and aligned, the noise returned. I discovered that I got the noise with no pads, as if the rotor were rubbing the caliper. It occurred to me that each time I aligned the caliper by squeezing the brake lever, the caliper could move a little farther to the side of the faster piston. I retracted the pistons and found that the gap between pads was 0.8 mm wider than the rotor thickness. Before aligning the brake, I put a playing card 0.3mm thick, on each side of the rotor.
It still rubbed. Then I noticed that a few inches from the brake, the tail of a zip tie was rubbing the tire. I'd put that zip tie around the shock months ago as a record of how far bumps compressed the shock; when the shock compressed, it would push the zip tie up. I don't know how it got down below the fender, why the movement of the shock didn't push it back up out of the way, or why it didn't rub while the caliper was off.