I just noticed, since this was before Rapiel (Son birthed by Aeroc) knew that Aeroc was his papa, we know that this was before the headstone was smashed to pieces and that Aeroc's situation must've been out and about as a gossip in high society. Maybe, as we see Rapiel glance back at the headstone and possibly hearing some gossips about the previous owner of the house they were living in, he must've questioned why a tombstone of someone would be there inside their territory, he must've had suspicions about it but kept it at the back of his memories, and then continued to hear more as he grew and brought the pieces together, and came to the conclusion we have already seen.
If I'm right that'd be wild
This story just don't let me live in peace
First timeline hits right in the feels and fists