Hi Ian, this is a bit of a puzzle eh? Hubby and I have been having a bit of a think about what it could be and we wondered if it was one of a pair of reflectors that was mounted into e.g. posts or walls to indicate it was OK to proceed past that point - indicating an exit from a car park or similar, or maybe a reflector for foggy conditions? At 9" diameter it is clearly meant to show up from a distance away. Neither of us has come across anything quite like it before though, and our thoughts might not be at all helpful!