There is a company that does inspect the oil, I can't remember the name but it is a company that the diesel drivers use. I expect that BMW has already discarded any oil/evidence of the failure.
It is strange that this "mystery metal" issue has just come up again in another post.
I think the noise difference in the engine sounds like a top end issue or maybe some shrapnel is clogging up the catalyst as it gets pushed out of the cylinders. That probably would show up as a O2 sensor error and a check engine light.
Also, if the service tech scoped the cylinder they would not see much other than the top of the piston and cylinder wall. Most of the scopes don't have a head that can look backwards at ~180 degrees to see the valves. Maybe a fancy spy scope would do that
Or it could be a bit of aluminum stuck between a valve and the valve seat?