Brake warning light on (but very dim)

[caveman_si]

At start up the brake warning light for the car comes on normally ie as bright as all the other dash lights that come on when you start up. But when the other lights go off the brake warning light almost goes off but stays on but very dimly lit. its almost not noticeable in day light but in the dark or if you shade the dash the dim light can be seen.  It doesnt change when you change the dashboard lights brightness. 

Car has had recent brake discs/pads and fluid change and the fluids are at correct levels (I have double/trippled checked the levels). And there is nothing wrong with the brakes in how they perform. The warning light isnt the bright light I would expect with low fluid levels/see at start up.  I think the dim light flickers just a smidge when I turn on/off electrical loads, radio/heating etc.  The dim warning light goes off if the sensor in the brake fluid reservoir is unplugged.  The car hasnt been driven much recently but had big runs earlier in the month, the Battery is down around 12.3-12.4v  but even though its been chilly the car starts fine and there is no other hint of electrical issues (touch wood). 

My thoughts are its probably electrical and its likely its the brake fluid level sensor has gone partial open circuit/failed . Or it could the low Battery voltage is causing issues, I also wondered about a brake light being out but I am getting light from both sides and high level as checked against a wall. I havent had time to check if any individual leds are out on the high level rear brake light but the light looks even.

Does any one else have experience with this type of issues as when searching cant find anything similar in symptoms ie dim brake warning light or reference to brake fluid level switch/sensor issues for the avensis. Any ideas on how to test the sensor eg resistance measurement etc to confirm its the sensor. Also any chance anyone point me in the direction of part no / parts diagram for the sensor/ reservoir so I can get one ordered. 

Car is a 2013 avensis d4d. 

[SpyrosG]

I have the EXACT same issue on my 2009 Avensis! I would't call it a "problem", as everything is running smoothly, but  it's been 10 days that I have noticed this very dim, very faint, red exlamation mark that won't go away. @caveman_si, I am wondering if you have figured out the cause of the issue in your case and how to fix it.