So, there is no right answer here. All the options will in some way have negative effects. Your parents' relationship should justifiably be afforded some level of privacy.

That said, your father just brought it to his kid's attention by not coming home one evening. In effect, he made it your business. I would be so pissed at the man - he purposely put you in that position. What a selfish thing to do - now it gets to weigh on your conscience, not his. If he is in fact not cheating, he owes you an explanation as to what-the-h3ll! It's out of the ordinary and I would have been worried about him being a no-show all evening. Car accident?

If you are going to say anything to anyone, start with him. And if he confesses to an affair, give him h3ll - how dare he put you in this position. Then, I don't know . . . If he was adult enough he would be working on things with your mother directly so you don't have to witness all the dirty laundry.

Frankly, anyone who cheats is going to get caught. It's inevitable. And, people who cheat KNOW it, too. Maybe he is ready to "be caught" cheating to get things moving between him and your mother.

You could keep your concerns under wraps - you have no obligation to speak up, nor do you an obligation to keep it to yourself. But not speaking to your father privately about it could really build resentment in you. But, you could be resentful even then. Your mother already suspects. This is a bad situation all around.

Anyway, from my POV, the bottom line is that if something inappropriate comes to the attention of one person by the offending party, then there is no obligation for the other person to do anything one way or another. The offending party is ultimately responsible for brInging down the house of cards.

Good luck with this.