The Taiga: After ziplining out of the Pioneer settlement, you will have to go through a cave. A Pioneer's corpse will be lying on the floor about halfway through. The note should be next to it.
Guys, I'm at the lower farm, with about thirty bandits. I've been leading them from the road, will hold them for half an hour maybe, losing too much blood. I'm sending the little one back with this message, 'cause he wouldn't leave otherwise.
Good luck, guys!