I'll give that a go on the next CD run.

Prof_Pixel:

It spent more than an hour in first dev without agitation, the thiocyanate has enough time to remove any remaining halide. The visual appearance is monochromatic, and not coloured. Looks the same as T-Max that went through E-6 once. Among other things. Ron suggested Silver Sulphide.

Bleaching back didn't work. After treating it with aluminium foil and an electrolytic solution, bleaching back has worked (that also worked on the sulphide treated test strip).

So Ron's right in that it was silver sulphide. We makes me worry the sulphur came from the thiocyanate.