[edited to add more examples]
Along several streets in London I can see paving slabs that have become stained in the middle of the slab with white-ish marks forming different shaped patterns. The staining is permanent, all year round and not dependent on weather conditions or time-of-day.
Typically neighbouring slabs have similar patterns, in a small local area. This suggests it could be due to something underlying the slabs - possibly the Reflective Staining effect suggested by Keith McClary.
Does anyone have any other ideas what could be causing these shapes?
(These are public streets so not possible to lift the slabs to examine what is underneath.)