Advertisements I got the impression they knew about the prison, but had discounted it previously. When Daryl and Rick were walking and spotted it, they didn't say "hey look, a prison!" Daryl kind of glanced at it and said "what a shame", or something to that effect. It seemed to me he was implying "what a shame it's so overrun." I think they knew about it but hadn't wanted to deal with it before. But now they were getting a bit desperate.