[stoppingdown]

I'd like to do that, but first I want to exclude the possibility of fungus; if it's a fungus I want to keep it at a distance from the other pieces of the equipment.

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I've done a bit of research, but unfortunately there are just very few images of polarising filters affected by fungus (notably one guy had his Sing-Ray [ouch] filter affected: https://www.dpreview.com/forums/thread/3358596). But I think that what I'm seeing are both residual of water evaporation (in shape of particles) and pieces of fungus (in shape of segments).

I couldn't be able to guess when those polarisers were last used, with the exception of the 77mm, November 2011: a couple of years later I moved from Nikon to Sony and that filter was no more used, being no more compatible with the new system lenses. It's likely that the other two filters weren't used much more recently — so it's more than ten years.

I think that the problem was probably caused by the fact that B+W filters were stored in their plastic cases that are more or less sealed. Probably I opened them for a periodic clean up in a day with high humidity, then closed the cases; humidity got trapped; then temperature dropped, water condensed into droplets that found their way inside the junction. Actually traces are in form of a uniformly thick ring near the border. Water allowed fungi to develop. This scenario is a bit forced, but I can't imagine anything better.