I have seen the same thing on a few irises. Nothing else seems to be amiss. I feel of all of the fans and nothing is in the leaves and nothing is wrong with the rhizomes so I note it and cease to fret about it. I have seen it in other years also. As you say it is not wet or sticky..... just a wet looking spot. I should try to wash it gently and see if it comes off. hmmm......
