According to the map I'm missing a treasure chest in the room inside the water tower (it has a ladder and a rotating switch) but I can't even see the damn thing. There's a blue sign with two letters on it but I can't figure out what it means or if it's even relevant. Suggestions?

I think that one might have to do with a treasure map? Not sure. Probably totally making this up.

@bwheeeler said:

I think that one might have to do with a treasure map? Not sure. Probably totally making this up.

I'm pretty sure you're right there sir. The treasure is out of sight and only obtainable through invisible ledges. There should be a treasure map that describes it better than I.

The treasure is way above you. There is a a set of invisible platforms you can use to reach the top. Or, if you're in NG+ you can fly up to it.

Yeah, minutes after posting it occurred to me to check the treasure maps. Thanks anyway.