The outfall for this drain looks a bit like STD & I was hoping for a similar set of features.
A set of stairs drops down from a small dry RCP and joins after the outfall, the whole setup looking like a 'Y' from above.
It turned out to be a straight CMP a few hundred meters long that eventually shrinks to a 2-3ft concrete pipe. A small manhole chamber houses the shrink and rises up a couple of levels. I ventured further up the pipe a short distance but was confident there was nothing further to see. The infall for the drain is a small grate a few hundred meters further up the valley.