I think the guy was just bored.. He'd driven past the other way about 10mins previous, then pulled me and said ''it appeared'' that I was not wearing a seatbelt. I'm not sure how good his vision was but I find it hard to keep my eye on the road and check road users for seatbelts - going the opposite direction at 40MPH..
... I just told him that I was, and he didn't argue it at all.. Then did a 'routine check' and let me go. Of course I said 'Thank you officer', but to this day I can't work out what I thanked him for..