Irisa and Demion openly showed off their intimacy since that day. There was no point in hiding it. Demion felt like keeping it a secret from Escus would only serve to make him feel like a fool, especially if he should find out about it in other ways.
Naturally, he didn't know that his fellow guard already knew and was boiling with rage. Rage from jealousy and from something else he knew.
Escus might not have been the most honourable man in the world but while in the company of someone he viewed as incredibly noble like Demion, he had been forced to conform to a more civil persona. In a way, he respected Demion and saw him as something of a moral compass.