This sign currently resides at a customer's house in the Springwater area.  Next time I'm out there I'll see if I can take a look at the back of the sign for any more clues.  One possibility is that is was at the intersection of trails 516 and 514.  The mileages would be about right.

The other thought I had was that the sign was made as some sort of prank or inside joke.

I seem to remember the subject coming up a few years ago and a discussion happening in a thread on this forum (haven't found it yet).  There are still remnants of a concrete structure out at the spring that appear to have been a swimming hole/pool.  IIRC the church relocated their resort to a location in the Columbia Gorge.  The area was pretty thick with young trees last time we were out there, but with the recent fires there might be more remnants visible now of the old resort.