SPOILERS ALERT, at the end, and at the start of "Irreversible" (2002) for that matter...

... The Butcher character reveals that he was arrested and spent time in jail for sleeping with his daughter, but how did the police ever find that out and discover it?

Did the daughter decide to report him and then turn him in, and was she carefully examined in some or other ways so that they could determine it WAS him who had the contact with her?

Or maybe he himself felt bad about it and decided to turn himself in, but given how he had no problem whatsoever escaping and running away after beating up his pregnant wife and causing her to lose the baby, as long as he doesn't get caught, why would he all of a sudden gain a conscience and decide to turn himself in to the police like that?

Or was it discovered somehow else? It wasn't quite made clear, and yes, it was revealed right at the start of Irreversible in the room during the brief exchange of dialogue, but does anyone have any idea of how it happened? And also, how long did he spend in jail for that?