I think it's almost like Baader-Meinhof Phenomenon. When I randomly recognized the first one, I suddenly realized there were many more I would pass everyday but had never noticed.

I didn't know of them until after I had moved to Hamburg, where there are quite a few. My SO showed them to me and told me the background.

I was flabbergasted. Being very interested in history, having studied history at university and never before having heard of them was quite frankly disconcerting for me back then.

Since I nearly always stumble (as intended) as it is a visual pattern I now instantly recognize.