I saw a similar sign out on a trail a few years back, near a fishing hole just below some rocky rapids/falls. If I had to guess the missing kid probably was at the slippery rocky ledge overlooking the top of the rapids, lost his footing and fell into the churn, and lights out.

The sign had a change of clothes, bottled water and some granola bars in a ziplock bag. You could tell the sign and supplies had been there for a while.

I think about that sign whenever I get the urge to try something risky out in the wilderness