Found 'Em

     The drain openings in the sunroof hatch well were obvious.  A little compressed air seemed to help, and some water-bottle testing on both sides resulted in water running out the underside end.

     The passenger side one was a little blocked at  first and cleared up, so I'm hoping that will cure the problem -- that, and making sure the sunroof is closed.  The controls are strange to me, a pair of spring-loaded, three-position, center-neutral "slide" and "tilt" controls that don't seem to have a positive hatch-shut setting: run them to the end and the hatch does tricky tilt-and-lift moves instead of stopping.  Time to read the manual.  Perhaps I'm missing something.  (Note to Lexus: Please don't design airlock controls.)

     Time and again, I've found that questioning my assumptions and checking the verifiable physical evidence and original documentation leads to a solution.  I'm sure there's a wider lesson there, but I'll leave it for readers to work out.