Steve: if you purchase a used optic do you check the serial number with the manufacturer to see if it's a gray market or USA warranty item?

When shopping for some photography equipment some time ago I learned of gray market directly from BH photo-video and it agrees with your overall assessment.

http://www.bhphotovideo.com/find/HelpCenter/USGrey.jsp

But seems that if this gray vs USA warranty issue was that cut and dry then you would hear some occasional complaints about other manufacturers like Swaro but the usual story is : "I picked up this old SLC from an estate sale, sent it to Swarovski, they fixed it up for a minimal charge." Never a mention of gray market or where it was originally purchased.
Could Leica just be more stringent in their monitoring of gray market items?