[jmciver]

So I know this thread is old, but I figured I would add another data point for the sake of others.... My expansion tank looked exactly like the OP's tank with dried coolant residue at the seems. It took a little pressure on the dealer to get a warranty replacement tank since my car also passed the pressure test. Fortunately, I did not have to wait for the seem to start "spewing" in order to get the repair done. Basically, my dealer went to bat for me as BMW NA originally wanted to deny it since my car passed the pressure test.

Just had the job done at 45k miles. 2017 M3 with a Nov 2016 build date, purchased in Dec 2016. So I think the moral of the story is for S55 owners to begin monitoring their expansion tank at least by the 40k mile mark. At least the tank is not that expensive or difficult to replace should a DIY be necessary if outside of warranty.