Thanks Constantin,
I think the phasing creates infill walls.
I have changed it to a french door from the revit library
See below - existing - windows. proposed - french doors:


I thought the problem arose when I add the curtain wall element with the RevitCity made folding doors:


But it still creates these infill walls. i think it needs them so that the phasing has something to "hold onto", so that it knows something is there in a different phase but it has to show a wall instead of a hole. It shows a line sometimes. below I have deleted the french patio doors and the infill walls remain:


Existing Plan:

Existing & Demo Plan:

Proposed Plan:

3D existing:

3D proposed:

3D proposed with the wall highlighted:

The white rectangles are not selectable but are wall created to infill where the windows were in "existing" because this is an existing wall with demolished windows
These infill walls are a bit of a problem because Revit creates them when I am editing things, like moving levels in section other odd bits of editing. I don't want them!