Another place to check , rear wheel arches, where rear clip holds rear bumper to arch . I’m presuming this is a common rust point, luckily this is only on underside of arch and isn’t visible with bumper fitted, so carefully sanding back and will treat light rust , and repaint seal etc,, there are a fair few cars that have been done under bmw warranty in this area, mine being one of them wax treated on the inside to prevent any problems, but I guess they don’t do this part of paint ,,
Glad I found this before it affects visible areas
