How did Rick find out about Shane and Lori because Lori didn't exactly confess

At the end of the episode 'Secrets' (season 2 episode 6):

*after Lori & Rick argued about her taking the pills*
Rick: Is there anything else I should know about?
Lori: Shane and I...
Rick: *sigh*... I know.... Of course I know... bạn thought I was dead. The world went to shit and bạn thought I was dead, right?
Lori: Yeah.

So this is when she told him, but he had figured it out on his own.