I see now that this image by James Adams (specimen collected in Georgia?) has uniformly light brown forewings like mine.

All-Leps lists two subspecies: S. i. insularis and S. i. julitae, so maybe the nominate subspecies is the "typical" whitish one with dark streaks found in the north, and julitae is a light brown subspecies from Georgia and Florida (?)

Your photo is the first I have seen from the south. Adams did not mention whether or not it is "typical" there. Truth be told, the Canadian photos don't show really strong markings, and I can bring out the darker areas a bit more in your moth by increasing contrast. Still, it is not what Covell leads you to expect.

Thanks, Bob. I spent probably too long looking for a match on the web and couldn't find anything decent, so an introduced species sounds plausible.
There were four of these hanging around a gas station; the others looked virtually identical to this one and the first individual, so whatever they are, they're consistent.