I'm so sorry, I just couldn't think of an interesting response and then it dragged on for this long
[He's taken aback by the apology and didn't think it would be so easy to get his feather back, so it's a few seconds before he even reacts. He doesn't kneel but he does bend down to take the feather gently from Syaoran and when nothing further happens, tucks it into his pocket for safe keeping.
He doesn't know if there are any consequences from losing the feather... better not leave it to chance.]
What did you see? [Ciel knows what he saw but he wants it confirmed anyway. It's still a visceral violation of his past, and it's better to know now how deep that breach goes.