I have always been able to keep my white plastic trays quite clean. I always wash them out & wipe them down, and have always used tongs to handle prints. Recently, I have done some batch printing of proofs where I would to 10 prints at a time, and I handled them with bare hands. Cycling thru them moving each from the bottom of the stack to the top.
Lo and behold, my developing tray now has a dark stain. I presume it is from the oils on my hands, as I have not changed developers and the only thing different I can think of was that my skin was in the developer. I tried soaking with a warm bleach solution, with no luck.
I suppose I could quit worrying... actually I am not worrying, nor am I excessively anal about keeping things clean, but it is interesting this happened.