I happen to have a small driveway which is covered in stones. These provide an effective alert that someone is approaching the front door (as well as ending up on the lawn and inside the house).
For those wealthy enough to have block paving (and therefore presumably with something worth stealing) there are numerous magnetic, infra-red and laser beam security devices, not to mention the traditional, roll-over surface pressure detector.
Today’s invention seems like an obvious improvement in that it consists of a wired network of pressure sensors each mounted beneath a paving block. It might be possible to have blocks each with a built-in pressure sensor. Only a subset, suitably arrayed, would need these.
As the blocks are very slightly displaced vertically when walked or driven over, so they can issue an alarm in the home or to one’s mobile -without the knowledge of a visitor.
A sufficient number of sensors would be capable of determining whether a group of people had arrived or a vehicle (and whether it left more laden than it entered).