Then we went through a very dark area where the natural light is obscured by scaffolding and some huge boards, making a kind of graffiti alleyway. Sure enough, her red light went on again, and soon turned off once we'd got into the full light.
Sherlock began to think: aha - she's got some kind of a cool sensor thing on her light.
My investigations led me to Google. And then to this link, where I found the "Busch & Muller Dtoplight Senso Battery Rear LED" (£27.99 from Evans) which I think is a match to hers. I must confess I didn't know this kind of thing existed, and now I'm a bit envious as I love all the gadgets and lights and I'm a firm believer that you can't have too many on a bike.
Anyway, here's the link for those interested: