I haven't heard of anything systemic so whatever it is, you may be the only one on the forums to experience it. I think you're going to have to do some investigation...
How big a spot is it?
Have you driven through any abnormally heavy rain, puddles or flooded areas?
Do you park it outside and someone left you a surprise?
Could someone have spilled something and not told you?
Look under the car for anything obvious.
Post a picture if you can.
2010 E93 335i M LeMans Blue
2001 E 46 330ci Coupe/silver/black/Sunroof
1999 BMW R1200C Cruiser-Ivory & Blue