Likely because he doesn't know/didn't remember the answer (though he obviously looked into Sara's murder notes, most likely out of personal curiosity) or far less likely because Jordan has put the resolution of this on the list of things to answer through AMOL (not impossible I guess as it's almost certainly Fain who arranged for Barthanes to be killed as soon as he had delivered his message. It's Fain who wanted Rand to follow, but the Shadow to lose Fain's trace. Another who could have a motive to make her masters lose Rand's trace but probably not the means to commission the murder without even leaving the party would have been Verin).

I've been to Q&A with Brandon, and he did say most of his RAFOs hid even less than RJ's. One just gets tired of saying "I don't know and I'm not sure there's an answer even in the notes" or 'I'm not sure enough it's safe to answer now or if Harriet might have wanted it for the Encyclopedia, or if it appears or not on RJ's list of things he explicitely wished left unresolved forever etc.". So RAFO stand for those answers too.