Ah, I vaguely remembered we had more info on the night with Phroneos somewhere, thanks for bringing it up and quoting it here for us, ChronicQuery!
Super interesting find on the link there, I honestly wouldn't put it past the dev team to go that far with something being called a star and meaning a pattern that tends to appear on vultures, lol. It's also very poetic, Asterion is just a poetic person and would think that way, and stars are already a theme between Luke's interests and Asterion's name. And THEN there's the fact that, when Nikos sees them, "his eyes are filled with both puzzlement and a distant familiarity", so if they really are griffon vultures and are seen in Crete, it makes sense why Nikos would recognize them since he's from there, too.
Yo that point on these bird sightings being around the confession or divulging of relationships would be fire if you guessed right on that! If the Master does not call out Nikos right away when presented the option (they can pretend that Argos is from the depths of the Labyrinth until he starts slithering away, then call him by name of Dominikos and lead to a more "Oh so you understand the role is fake but we're still playing it, then I'll keep my secrets to play the role out" encounter/aftermath in the Hotel in Chapter 18), there are no signs of the vultures, so I think you're on to something.
On the sacrifice, one has to wonder if its status as a sacred bird being sacrificed for a hero who went to Elysium meant it was not a taboo, or if it did indeed negatively affect (some of) the gods' opinion of Asterion. Was the fact they never saw each other again a punishment to Asterion, potentially from the gods? I don't think Pheroneos said anything bad about Asterion for hunting the bird (there's space to read the quote as Pheroneos turning heel and calling him 'Monster' instead of something nice, but why would he help with the fire for the sacrifice, then?), else I don't think Asterion would think of him so fondly... so I think calling him 'Brother' could be it, and could be a big enough breach to his job to get him pulled from the Labyrinth guard duty even if the gods weren't mad at Asterion...
Well unless it was supposed to be a confession of love, which would have the same problem of potentially getting him pulled from guard duty for. Asterion drinks to Khenbish's question in the drinking game about kissing after a physical activity and claims it's just kissing siblings on the cheek after play fighting... but kissing Pheroneos after hunting together with him is still on the table òwó (for the record, I think it's unlikely, but had to throw it out there, haha).