I'm still only on Act II, but I've talked to Sagani a few times and she keeps bringing up my search for Thaos after we talk about her search for Persoq.  The only problem is, I can't for the life of me ever remember reading anything about a Thaos other than when she says it.  Is this just a case of Obsidian getting the timing wrong and she's saying something earlier than she should?

I had this same bug. I can only imagine that the devs expected you to be further along in the main storyline before you encountered Sagani. A few other conversations "jumped" my progress as well.