Otherwise you can build the mesh with extrapolation enabled and that will extend the mesh allowing the photos at the extremes something to be projected onto, although the results are a bit unpredictable, it's only really useful for flattish surfaces.
Another option is to export the mesh into blender or equivalent and add a bit of surface there manually, but as JMR says if it isn't in at least 2 photos already then there's no way to reconstruct it without some manual/guess work.