Hi, first post on here since purchasing my used Z4 in November. 6 months after purchasing the car I too have the same rear window issue (some black coming away from glass on bottom right had corner) . Took it into dealers (have approved used warranty). They hadn't seen this before and their diagnosis (which I hope is wrong) is window coming away from roof and therefore requiring new outer roof. They have asked BMW for a goodwill gesture and will notify me in due course. From looking at your pictures mine is exactly the same and it is watertight. I suspect it is not coming away (no sign that i can see of that) and that it is delamination and just cosmetic - but very very annoying. Does anyone have any updates on their rear window issues ?