It's possible that the trail junction wasn't obvious except if the sign was standing up. I've seen junctions above treeline that were pretty much invisible because the side trail was so seldom used.

Of course he should have been expecting a junction anytime now... but I've walked past junctions on occasion. (Didn't continue TOO awfully far afterwards though.)

My buddy and I came across a fellow way up on the tundra in Rocky Mtn National Park who was doing a solo trip and had forgotten his compass. He wasn't lost yet but knew he needed the tool. So he asked to borrow one of ours. I loaned mine and when his trip was over he mailed it back to me with a note that said he'd had a great trip. At least he knew what he needed!