Yes, I also think this! RA came forward very early on and we don't know exactly when, but it's my opinion that it was very likely before the girls bodies were found. He could then tell people that he was there that day walking and didn't see anything and he even talked to LE to tell them what he knew. Those people he might have talked to about this would then assume he was ruled out or that he was just being the helpful guy who happened to be out walking that day. Maybe over time his behavior was suspicious enough that they started to think he could be BG.. BUT he must not be because he came forward and he hasn't been arrested right so they put it aside and carried on with life. I could absolutely see a scenario where someone called in and said this guy RA has changed his behavior and his appearance has shifted. Insert whatever other things they noticed here... AND then they say but he came forward and talked to police so I assume he was cleared..
Well that would likely make LE go back to the beginning and starting looking at anyone they talked to early on and looking for a report that could have came from someone named RA.. then they find the tip, but it had been misfiled.