Can anyone offer any advice?
Still battling with this. It's definitely not leaking. I've had a multiple water hoses running full whack on the roof at 1m intervals and the patches didn't appear, bone dry.
I did however get a borescope through the membrane to where the patches were showing and noticed drips coming from the back of the GSE tray and onto the membrane, which seems like a condensation issue. This is a new build, ventilation at eaves and ridge, breathable membrane but no insulation has been installed yet to monitor the issue.
I have since insulated between 5 of the rafters as a test to see if the patches disappeared, but they came back next time we had damp weather. Again, patches only show, at a continuous height all the way across the panels and in wet weather, I'm out of ideas.