Well this is annoying. After believing it was all sorted, I realise it's not, not really.
Around 3.30 I took the car up a very tree-lined road to visit the bottle bank with some empties. On the way back, at the point where the trees were thickest, the bloody lights came on again.
I didn't have time to go right back, but I returned about two hours later, minus the new sensor cover. All fine, lights not coming on. So I thought, time of day, need another run with the cover on. I stopped under the trees to put the cover back, and immediately I did that, the lights came on. OK, not good, maybe it's the material I used to cover the holes, perhaps that's not as transparent as it should be. I'd done a better job glueing it on than I realised, but I got it off. Same thing. Put the cover as printed on, lights come on. Take it off, lights stay off. This is a bright sunny afternoon, and nobody else's lights are coming on under these trees. It's embarrassing.
I took the cover off again and did a complete run up and down the avenue like that, and it was perfect. Lights stayed off.
To my mind this really needs a completely transparent cover. Not quite sure what I'm going to do in the interim. I can't really drive around with the sensor completely naked, but that seems to be the only way it's working correctly.
How about you, Siteguru?