Full context:

A spider's symbol was found on a telegraph pole in the middle of the heartlands, overlaying this symbol on top of the map leads to 8 different telegraph poles, each with a strange spider web and a feather attached to them that can be shot off.

Heading to the center of this symbol, you will find a tree with a massive spider web with the letter "N" and a drawing of a pole clear on it.

Heading straight north to a telegraph pole from this tree and shooting that pole will reveal a completely secret message under the wood that seems to say you must head west down 5 more poles.

Doing that and shooting that pole reveals the final message, A guitar symbol and the letters "NW". Right now, this is where the mystery has reached a wall because heading northwest leads to Fort Wallace which has a few guitars but none of any significance so far.

This is the biggest discovery in Red Dead Redemption 2 in years and it's only just the beginning, no resolution rn.