Likely it depends on which Triumph ... on my Triumph Stag, it's the wire to the "key-in-switch warning buzzer". It gets grounded when the key is inserted in the lock (not necessarily turned, just inserted). Then the other side of the buzzer gets power whenever the driver's door is open, so the buzzer sounds if you open the door without taking out the key.
Or at least that's the way it did work, before I got annoyed at it /bcforum/images/%%GRAEMLIN_URL%%/grin.gif