In that case it does make sense to make the assumption it's "greenfield". 

The aim of my note was probably just to figure out the extent to which greenfield was used by mistake to tag "green fields" and to see, when used correctly, if any context could be added - for example a note linking to a planning application.

Do you think it would be fair to retag this as [natural=scrub][landuse=greenfield] (both tags) and add a fence spanning the perimeter?