Haa, yes fixer smell. I don't "leave" the trays out like a school darkroom. Soon as a session is done, I pour the chemicals back where they belong and dump anything that was used as one-shot. So I expect clean smell.
I forgot that I also addressed another possible source of the mildew smell... And since the original smell was "under" the sink, I think this was really the problem. I made a wash table from old backyard wood furniture, that lifts the wash tray up high enough to use the Kodak Tank and Tray Siphon.
This little table gets wet. And when I'm done, I put it under the sink on the concrete... It HAD started growing fungus. Not quite like the moose-antler fungus I find on trees in the wild, but white chunky stuff that needed to be scraped off. I also treated the table with a little bleach after scrubbing off the obvious stuff.
But at the time that I smelled the musty odor, I just pulled everything out from under the sink... And I assume when I removed the table, I removed the smell...
At least I don't have any buildup of water and odor RIGHT NOW... And I will keep my fingers crossed that things stay this way.