This is from the mechanical drawing for the XSON6 package in this datasheet (page 13).
What do the gray rectangles represent? Note that they don't have any measurements.
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See note 2: Can be visible in some manufacturing processes.
My guess is that there may be some exposed copper in this area.
They will be the trimmed off extension of the internal contacts which extend out of the plastic at this point on the leaded versions. Plastic not thick enough it appears :-).