I'm not sure there is a sensor. The ECU measures various parameters like vehicle speed, gearbox (gear) and clutch pedal position, battery voltage, whether a/c or the rear window demister are on, engine temperature etc. and decides whether or not to activate the S/S.
If the car is stationary, in neutral with the clutch out, up to operating temperature, with a spunky battery and no a/c or rear window demister on, then it'll cut the engine. Anything else, it won't.
If your car isn't cutting in the S/S then the battery is probably the wrong type, or not at "peak". Early S/S and non-S/S cars had a weedier battery that loses the S/S mojo quite early in its life. If you fit a new battery of the later S/S type (which are twice the price, hence a lot of second owners fit the cheap one) then it should re-vitalise the S/S.
If you trickle charge the old battery off the car, and clean up the battery posts with wire wool (don't short wire wool out across the battery..
) and fettle the insides of the connector clamps, then the S/S may work for a few days until the battery fades below the threshold again. A non-working S/S doesn't mean the battery is no good, just not A1 condition, so it could last another 4 or 5 years yet...
The battery condition sensor is next to the earth battery post. If you charge the battery, make sure you attach the charger to the earth post, not the bracket on the other side of the battery condition sensor (otherwise it'll get burnt out) - or (some models) use a separate "falso earth" charging post, which is nearer the front of the car. If you have three terminals on the battery, use the one without the sensor attached to it, for the earth.
Ralf S.