What...? [He hadn't noticed it, so he's looking at Porthos with a comically puzzled expression... and when Porthos touches the feather, it'll show a conversation he had with Arthur not too long ago, up until Arthur tells him to shut up.]
[Which means Aramis himself is recovering that memory, and whatever he was going to say about the feather is forgotten. He's looking horrified now, color draining from his face.]