S-Bahnhof Friedrichstraße

From the scan, that doesn't look like fog, rather it looks like water spots from drying the film after processing.

Yeah, that's water spots.

Water spots or not this is a fine picture, making as it does a strong social comment. The suited man striding towards his well paid job is determined to give the busker playing the accordion as wide a berth as possible; too close and he may feel obliged to part with some money.
Soaking, refixing, and rewashing the negative in distilled water may remove the stain.