Elsewhere in Toyota Owners Club postings I read of moisture leaking into, or condensate forming in Sienna passenger side headlight. Had this happen to me at about 18K while touring central states.
My dealer service rep said this was a Warranty issue (car mfg date 2-2-2004). After inspection to check for scratches, damage or evidence of non-dealer repairs, the headlight unit was removed and replaced. The faulty unit was returned to factory for further inspection/analysis.
While in shop, my dealer checked w/ Toyota Technical Service and found two "recall" issues that affected my car. Performed the specified repacements.
This is mighty fine service. Maybe this post will help others. :)