By the time Gladio stops speaking, the stillness that previously only affected Ignis' fingers has spread to his whole body. It doesn't even seem as if he is breathing. As if, in just a few seconds, he's been turned completely to stone. He only knows his heart is still beating because it hurts so much. First, to have Gladio think he was purposely keeping secrets again, and second, realizing that Noctis saw...

Breath finally shudders from his mouth, those shudders traveling down to his fingers as he searches for words. It was never meant to be a secret, but it had come out in a conversation where he and Ardyn talked of so many of his sins. Ignis had not been able to process all of them right then. He only knew that none of the truth changed what he felt--what he wanted--so it was pushed aside until he was able to process it.

Apparently, he waited too long.

He steps back from Gladio, unconsciously trying to hide how shaken is really is.

"Noctis was unconscious when Ravus and I made it to the Alter. I didn't know he had witnessed any of it."

It's not an excuse. Instead, Ignis is desperately grabbing at some starting point while he finally processes this particular sin of Ardyn's.