Dunno. Has been in that field with the others many times before as per usual routine. The others show no interest in following. Quiet field, nothing but the sheep in there.
She certainly isn't thin, the grass is good where she is, but suppose can't rule out 'the grass is always greener on the other side of the fence'. The adjacent field is ungrazed.
Need to keep her in though, or it could well turn into a habit no matter what field she is in?