On Reddit the following photo was posted: http://imgur.com/90ty52N
It seems the field containing the grey shepherd was fully enclosed by a different player. No more tiles can extend that field. Of course if the pasture was enclosed by the grey player they would retrieve their shepherd and score for their sheep.
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But what happens to the grey shepherd when someone else encloses that pasture?
In the BigBox5 from 2014 - German rule book is this situation in the rules as a "special case" (page 11).
If a player surrounded a farm complete with roads and citywalls, then the action "gather the flock" triggers.
Is it stuck there until the end of the game?
No, it isn't.
Does it return to the grey player scoring no points?
No, this suggestion is wrong.
Does it return to the grey player scoring points for the sheep?
Yes, this is right. The shepherd returns to the grey players supply and he gets the points for the sheep.
The sheep return into the bag.