Admittedly nothing to do with DIY, but someone here is bound to know or at least propose a feasible explanation...
Around York recently a number of strange items have appeared set into pavements. They are around the town center and are a flat ring of coloured plastic of about 10cm diameter with a raised half sphere of transparent material (presume it's glass) in the centre of diameter approx 3cm. These things are coloured either red, white or blue. The ring and the transparent half sphere are the same colour.
They are deliberately set into the pavement and must have been core drilled in. They are relatively sparse and not consistent with marking anything obvious (like shop boundaries or driveways etc).
They appear to be a fairly constant distance from the closest building or structure (some white ones are next to a wall on a bridge) being approx 1/2 a metre away.
So - fire away if you have any ideas. Appologies for not having a photo to look at, but I havn't taken my camera into town recently.
cheers