Do you do sulphide based sepia toning in the same room as you store the paper? The fumes can apparently fog emulsions.
No, selenium is the first and only toner that I have used so far. I have only been printing seriously for about 4 years. The residual silver test from photographer's formulary is made with sodium sulfide (pretty nasty stuff), but I keep it FAR far away from my darkroom--in a different building actually.