Can I suggest you use a strong light / lamp / powerful torch [+1000. Lumen] and shine the light beam not at the surface but hold the torch a few MM away from the wall and let the light cascade over the surface, then sweep the beam all over the wall.
Now? if you can get some images of the wall with the light sweeping over it.
When the above is undertaken it is amazing what [historic] information you can spot? simply by the undulations / old patches / old holes that you can see when the light is shone over a wall.
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