Could it be that the substance was tracked in from soil that they were walking across? Essentially efflouresence from the dirt on their shoes? Or is it on the entire surface and not just the walked area...
Is it heavier in some areas and lighter in others?
Also I have worked on commercial sites where I would finish the entry and the masons hadn't finished the outside stonework on the building and they would end up leaving a fine layer of 'mason sputum' across everything I did, which in turn the weather, (either dew or rain) would turn to essentially a waxy coating of goo that I would have to go clean up because they didn't feel like cleaning up after themselves.
