IMO they only confess the bare minimum.
I knew, just knew, there was a lot more my DH hadn't told me. I also knew that the key to it all laid in his -not so- secret email account.
I bided my time and four months later an opportunity arose and I took it and although the majority of messages had gone there were a few sitting in the sent mailbox that confirmed my suspicions that there had been someone else, in 2009.
I woke him up to ask him about it and he said he'd met her once whilst in London on business. He also confessed to another 'encounter' with a woman in a hotel in Liverpool.
I still wasn't convinced so dug around and it turned out he'd met the London woman twice, at a motorway services travel lodge type place, once for a shag and once (as she put it) for an all nighter.
I could see that they'd kept in brief contact via email and altho it looked like she'd wanted to meet again, he made excuses not to.
So, to answer your question, no, I don't think we ever do know everything. I know that I know everything about the London woman as I have spoken to her. 
The Liverpool one..... Have no idea who she is or when they met, he says 2009 but I have no evidence to back that up.
Are there any more, he says not, but given that ive never seen any correspondence re the Liverpool one, who's to say there's more that I haven't seen......
Are you still with your DH ?
Take care