Poking around the site a bit, I found this similar encounter posted by er0ck in 2011:
My observations were similar, except I clearly saw a trail leaving the creek on the other side....when i came down from nesmith pt on sunday i wanted to take 400 back to ainsworth.
i found where it looked like the bridge used to be over the creek from that last switchback on the nesmith pt trail. there is a sign post there behind a stump, and it looks like lots of people have gone down to and/or across the creek.
i found a better way across just down stream where the three rock stairs are on the nesmith pt. trail.
i saw no signs of a trail on the other side. i must have trudged around over there for an hour.
So here's my story of investigating these three trails.
It was the best of starts, it was the worst of starts. I wanted to begin in the dark and in the rain. Not because I like either, but for the novelty (I've hiked in darkness, I've hiked in rain, but not both together). But the drizzle stopped, and I was running a bit late by the time I hit the trailhead, and when I went to load batteries in my flashlight, the one I never use, the one I brought because it's my best small waterproof flashlight, I realized I had habitually packed AAA, not the AA I needed. Incredibly careless. That's the kind of mistake that can get your name in the news, under different circumstances. But I just had to wait a few minutes for enough light to sift through the clouds, and I was off.
Along the way to the switchback where the trail leaves the creek and starts ascending steeply, I passed the water pipes that cross the trail. They're maybe 2" pipe, and almost certainly the same pipe seen at two places on the trail. I could hear water running through them, which shocked me. I hadn't noticed that before. And you'd think an old pipe like that would be clogged or collapsed. At the upper pipe crossing, I followed it in the direction it seemed to headed (underground) and looked to see if I could find an intake at the nearby creek (not the other creek I mentioned above; this is just a little tributary). Found nothing. I also thought about trying to follow the pipe down to see where it went, but of course it could go anywhere, and I'd probably never find the end of it. Finally, the obvious occurred to me: This must be the source of the water at the tank down at the TH.
Okay, so on to the old signpost at the switchback. I was going to fill my waterbottle here, but the creek had gone dry sometime in the last week! Could hear water above though, so dropped into the creekbed and followed it up until I found flowing water, then back down. Poked around quite a bit on the west side looking for signs of a trail. Nothing. Moderately dense brush though. I figured even if I found something over here, it probably wouldn't be worthwhile trying to follow it. If it's this densely overgrown right at the start, it's probably not going to get any better.
Headed down to trail B (see map below) and followed it back to where I had turned around on my previous trip. Just past that point, I was shocked to encounter a junction. Took me brain just a few seconds to process this and realize trails B and C had to be the same. It was essentially a big switchback shortcut! Sounds foolish of me, but they were so far apart, and the switchback was relatively gentle, so in my mind these trails which headed off to my left as I descended Nesmith were basically headed westish (because I felt like I was hiking basically north, and the trails went off to the left). Another reason I should have realized what was going on here: I surmised that the two pipe/trail crossings were the same pipe; the trail had curved around to meet the pipe again below. Why didn't it occur to me that the same concept applied to my mystery trail? Of course, just to be sure, I hiked back uphill on Nesmith to confirm it would take me back to the top of the shortcut trail.
Got back at Yeon trailhead about 7:00, a bit annoyed and frustrated, having squandered a couple hours of dryish daylight hours on things I easily could have investigated on my previous trip. C'est la vie. Oh, and it turns out I never even needed that water I went hunting for.
