If the affair is taken underground, it means that it will continue and they don't want anyone to know. It could be only EA, no way to really know. But usually PA, otherwise why go to all the trouble of hiding it.
What did you confront your wife with?
Too many times someone will confront their spouse with only gut feelings or one little text message. And that is enough to get them to hide their affair much much better.
It is usually more than ego stroked, it is more like an addiction to excitement and or doing something wrong or something like that. That is why it is called the fog of the affair. No real logical thinking going on by the WS.
What kind of proof do you have.
You will need to watch and snoop to get hard proof for one thing. And then bring the affair to light. Affairs hate reality and the bright lights.