I wonder if RC "knew" that the coat had been in the woods for a while because JMB told him (per GG). The confusion and ambiguity in this case never ends!
I always thought that RC knew the coat was there because he (or someone; some other kid/searcher/etc.) had seen it there before the murders. This was a very popular spot, and I'm sure someone had seen it there before.
I don't know, to me, there's nothing suspicious about this coat at all.