In one of the streets near where I'm staying, there's a series of these mosaics replacing regular paving stones.

Some in the series are ordinary paving stones themselves, just of a different colour, but there's a definite regular sequence.

So, some guerilla paving artistry, or what?

Well, round the corner there are one or two more prosaic inserts in the paving: so perhaps these are, after all, no more than markers for the connection point between the house drains and the main sewers.