When Gaeta looks him in the eye, it confirms it...but he also sees what he needs to keep his silence.

To do something like that when it goes against everything? Maybe Gaeta did it, but no one who's truly guilty would ever feel regret for a decision like that.

He opens his mouth, then shuts it and starts patting himself down. He ran into it earlier when he was looking for his housekey that morning as he left, kept it to include in an offering to Serranai later on...

Finding it, John pulls out a single chicken feather from his pocket--shed by a very specific chicken--and hands it to Gaeta with an absolutely serious, absolutely sober expression.

"Swear on her life." he assures him. "This stays between us."