...they would have no way to know that Z.Soph was in the barn. ... I'd have to watch the early episodes again to be sure, but that was the explanation given by the writer last night.
This is a pretty crappy justification to me. If Hershel's people don't notice when the number of zombies in their corral changes... well, I guess that justifies all of Shane's fears about how dangerous it was to keep them locked up in the first place.