Why did I find the key to the cave?

Warning: The spoilers in this question may spoil the game plot for you.

Okay, so near the end of the game you find the key to the cave. It's attached to a backpack, which is tied to a tree,

which has an alarm that goes off when you find it.

I don't really understand why I found it. I mean, I know that

I used my signal tracker thing to find it, and presumably it was the alarm that set my tracker off, but Ned Goodwin spends the whole game trying to keep you from finding the cave. It is totally unclear to me why, right at the end, he apparently abandoned that plan and set you up to find the key, and thus Brian's body. This goes against everything he spent most of the game doing and when I later found his camp, I didn't find anything that explained his change of heart. In fact, I found a note about how he lost the key.

How did the key get there?

In Ned's bunker, at the end of the game, if you go to the side opposite the entrance (near the lawn chair) and look at the paper's that are hung on a clothing line of sorts, the final page near the bottom it has these lines written by Ned:

-2F (Meaning 2 Forks/Henry for short) Found Supplies -Keys Gone -Can't Let 2F find B (B for Brian Goodwin)

It also says "(HOW?)" next to the "Keys Gone" line, though it could be attached to the Found Supplies line. Either way it would imply that another party is involved and was the one who lead Henry to the supplies/key that was strapped to the tree with the alarm.

You could alternatively say that if that is just a red herring or a misinterpretation of the note, that perhaps he wanted to lead Henry into the cave and then when he locked him inside he assumed he would be trapped for good and die in the cave as well. That said, it is equally possible that Ned was just following him because it was a last ditch effort to get rid of Henry after he realized that he had somehow gotten the key to begin with.

In Ned's camp, there's a note about Henry ("2F", ie. "Two Forks Lookout") finding the pack, and the keys being gone. Ned also wrote "HOW?", implying that Ned didn't lose the keys, but that he was baffled as to how Henry found them.

The Audio Commentary Tour explains this: Ned hid the pack himself, including the key to the cave, as a bug-out bag in case he had to leave quickly (like, say, if there was a big fire, or if someone found him). He attached a home-made alarm to it, so he would be alerted if it was accidentally found by someone (like Henry). What Ned didn't consider was that the wave receiver would pick up on the alarm's signal, leading Henry straight to it.