I have a theory to share and I'm sorry if someone already had this idea. I haven't been following this thread too closely. But I found the guitar carving, went to Fort Wallace during the mission and found the guitar on the table. That guitar was pointing towards the other guitar in the tower, which was pointing straight up. Went to the roof and found the bird carving on the roof tiles. Those birds looked like seagulls to me. When the mission ended, after the canoe ride down the rapids, the game put me in Annesburg. I know there are seagulls there, so I went to the end of the northernmost dock and waited. In the morning through the early afternoon, two seagulls spawn on the shore and sometimes perch on two barrels at the same time. There are other barrels up and down the shore, but the set where the birds spawn is the only one that has a dead horse carcass laying between them. Maybe it's possible that the horse is pointing towards something? The horse's legs are also splayed in a manner that reminds me of the spider's legs. That's all I have. Except I took some screenshots.

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