It is the bulb failure light.

Double check your rear lights. If the tail/brake light fails, then the other tail light takes over the role of tail/brake light instead so you still have a full compliment of working brake lights (providing the lower tail lights haven't already failed)

ie, when everything is ok - from top to bottom

Indicator
Tail light / brake light
Tail light
Fog tail light (drivers side only)
Reversing light

But when the brake light fails:-

Indicator
no light
Tail light / brake light
Fog tail light (drivers side only)
Reversing light

Also, when a bulb is imminently about to fail it can also put the warning light on. Basically the bulb resistance changes enough to put the failure warning up.

Edited by Dynamic Dave on 07/09/2012 at 13:56