Apparently we messed up a bit with the sheep fence as one of the lambs found her way out. When we enlarged the enclosure at the weekend, we forgot to attach one part of the fence to a post, so there was a gap. Our neighbour called us to say that one of the girls had found her way out. Twice. Fortunately she didn't share her escape route with the others who were too busy trying to figure out if they could use the wooden steps to get over the fence. Trying to get her back inside the enclosure was, apparently, a bit of a problem and the whole flock took advantage of the situation and went AWOL.
Our neighbour fixed the fence and rounded up the flock.
We're learning fast.