My Brompton's lights have a peculiar characteristic, because of the combination of components.
SON front hub
B&M Front light with an on-off switch, and a standlicht (pardon my German) capacitor
B&M Rear light with no switch, and a similar capacitor.
When I'm riding, with the switch set to "off", then both lights are out (assuming the capacitors are discharged).
If I switch the switch to "on", and then turn the front wheel, both lights come on. When I stop, the lights stay on (although the front one dims noticeably) for some time due to the capacitors. During this time, if I turn the switch to "off", the front light goes out, but the rear red light stays on until its capacitor is discharged.
Normally, this wouldn't be a problem.
But carrying a Brompton with a lit red light on a station platform is one of those things up with which I will not put. (I got given a huge earful by a train guard on a Winter's evening in 1985 on Shenfield station, when my dynamo light lit up as I ran towards the guard's van, and the guard nearly mistook this for the platform dispatcher's hand-lamp. The lesson has stayed with me. Lights on Railway Premises are railway lights or none at all. Helped by my busman's holiday job as a signalman on a heritage railway line.)
So Diver300 (otp) helped me to fit a "discharge" button to the rear light, that shorts out the capacitor. If I press it, the light goes out. If I hold it pressed for 15 seconds, then the light stays out when I let go.
I successfully used this "discharge" button this morning, after the 2-mile bike portion of my commute, before climbing the steps to Willington platform for the 10 minute train journey. Willington was dark enough that a red light on the platform would have been a definite no-no.
Anyone else got lights they can't turn off?