Hi
Its not just fluid level warnings that can cause this...
I had exactly the same thing...master light came on but no other light to indicate which was causing it...
After a couple of weeks of it driving me nuts, my ABS light finally came on as well, which after a LOT of fault finding turned out to be one of the two ABS relays under the front wing....
So it COULD be ANY of the warning lights that come on when you turn on just the ignition....
And it may or may not be the coolant level ..... there is also a sensor for windscreen washer fluid level by the way...
All the best Brett