[In the midst of all the memory loss, here's some déjà vu: Sebastian stalking right up to a feather - Porthos', not that he knows - he's spotted, due wariness of magic glowy things be damned. The moment he spots the feather it all clicks into place - why he's feeling off, why the young master feels off, why the young master suddenly doesn't want anything to do with him... and Sebastian won't tolerate it. He'll turn the place upside down finding the relevant feathers if he has to.
So, yes. If Porthos doesn't notice the feather, he'll likely notice Sebastian having noticed it, seeing as he walks straight past Porthos towards it.]