Sometimes YOU are the last to know. Or you sense it, yet have no proof. Or you know it, yet choose to stay in denial. Or you find out and he promises to stop or change. Not likely in this life time.
Is your narcissist a sex addict and you don't know it or are in denial?
My narcissist never cheated on me is a common war cry on here.
Are you sure about this?
You don't need to SEE it happening to make it true.
Many narcissists are clever with keeping their sex addiction from you.
Often, you are the last to know.