If it was not ぬ but め, it refers to firefighters in the Edo period. This め is originally just a code name of a certain firefighting team, somewhat akin to Alpha/Beta/Delta (Team) used in English-speaking military.
Forty-eight groups were formed to the west of the Sumida River. All of them except four were designated using single hiragana letters and thus were collectively referred to as the iroha 48 gumi (いろは 48 組, "48 alphabetical groups").
め組【ぐみ】, which just means "め-group", has become the subject of many fictional stories after the Edo period. For example, see 暴れん坊将軍. The hiragana め has become somewhat like a nationally-known symbol of traditional Japanese firefighters.
Are there some other noteworthy examples of a hiragana character being used as shorthand for a point of interest?
ゆ and め are not "shorthands". ゆ on its own means "hot water", and め is just a code letter. There are not many similar examples, either. Perhaps many Japanese people know Hisaya Daikokudo's unique ぢ ("hemorrhoid") ad.