Years ago it became evident (or so we thought at the time) that the 2 screws that hold the exterior mirrors on can become corroded. Some owner’s even reported losing a mirror. The corrosion happens because water sits in the recesses of the mirror base after washing or heavy rain. I replaced the screws on this car with new stainless steel ones from McMaster Carr.
Now, all that said, the original BMW screws on my 16-year-old car were fine and showed no evidence of corrosion at all. But, My S54 has these stainless screws and it's better to be safe than sorry.