The confines of the field

[varefump]

Dan,

The definition of 'Bullpen' (Rule 2-8) is: "The Bullpen is the area in foul or dead-ball territory where substitute pitchers, catchers, and other players warm up."

Therefore, "behind the shed out there" would be considered the bullpen and thus fall under the confines of the field.

Now of course if the bullpen is 'outta sight', we as umpires would not be able to enforce the safety rules and there's no judge or jury that would hold us accountable in such a situation.