I picked up a used mig welder, helmet, jack stands and some other things, and the BMW overheated!
Good thing I was at the car parts store. The overflow reservoir was empty! After having my wife rescue me when I overheated a second time I found the culprit. It looks to be some of the hoses that run to the heater core. I found the 15 Euros worth of antifreeze on the ground at the back of the engine, near where the heater core lives. I also found it doesn’t seem to overheat when I make sure the heater is turned off. I assume some valve closes that prevents further leaking.
I’m frustrated, that’s all.