I find myself topping up the expansion tank on my 2017 540i about every three weeks now, maybe half a litre each time, and there's absolutely no sign of a leak anywhere on the garage floor or around any of the hoses and connections I can see from above. Had a pressure test done at a local shop and they said it held pressure perfectly for twenty minutes which just adds to the mystery, because the coolant has to be going somewhere but it's not escaping externally and there's no sweet smell from the exhaust that would indicate a head gasket breach. I'm starting to worry about the common B58 issue where the coolant leaks into the intake manifold through a failing integrated intercooler in the intake, or perhaps a tiny crack in the expansion tank cap that only releases vapour when the system is hot and under load. The car runs perfectly, no overheating, no rough idle, no milky oil, which almost makes it more frustrating because I can't just point at a symptom and say there's the problem. I want to take it to a BMW Garage that has experience with these mysterious B58 coolant losses and can do the proper tests to isolate whether it's the intercooler, the cap, or something even less obvious before I end up stranded somewhere with an overheated engine.