Found this on the first mission of HoW. After the Winter Walker where you get forced to leave the heavy pike. Left cave.
Wut dis. Y yu do dis.
Edit: I believe it is the House Judgement symbol. What this coin was doing there I'm not sure. Shooting it, melee, teabagging it, nothing affected it.
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Might be more dotted around. Wasn't there originally a coin hunt, with an emblem for the winners?