I don't think he's found out very much! He talked to Lily and kind of stumbled on Lily speaking to a ghost who had a similar experience as what was left behind by the undead in the Vannozza's forest. Otherwise he's been speaking to Wen Qing but didn't learn anything new from that conversation except that she was healthy as a child so this happened later, which makes him think there was a specific event or action associated with it. It also inspired him in general to look into Vannozza's family more and their history, including how her parents died, etc.

He had Emilia ask around with her connections (high society), and he is asking around with his own (scholar, necromancy) to find out more about her family in general/specifically related to death, etc. That might be how he could come into that knowledge? If he knew he was going tomb hunting, he'd ask around (even to Vannozza herself) about relevant tombs before hopping into all that.

But that is all very good information in general! He might be interested in just teleporting into Hatisse's tomb to see what's up, but I know it's all warded.

From this alone, I think he'd potentially be interested in investigating the underwater sorcery district as the next place to look into.