AirPort Utility not showing AirPort nodes, which ARE there, and I AM using them
Oddly rebooting the Mac seems to have fixed it.

Sure, seems like an obvious idea in hindsight, but at the time I was in the middle of a bunch of other stuff so didn't try that.

Also, since I was actually connected to it for Internet wifi, I certainly didn't think the Mac was having any trouble.

Well, if anybody else has this problem, it's something to try.