I have a 2006, automatic. I've had it sitting in my driveway for some time while waiting for me to replace the brake pads and rotors. It's been running fine since the day I brought it home shiny and new, but today for the first time I noticed there is visible exhaust. Not dark smoke, just wispy, fog-like light exhaust. Heavier if I rev the engine. This hasn't happened before. I did a search for "2006 mini cooper visible exhaust," which yielded nothing even related to my question.

What does this mean? Do I need to fix something?

 

Ca be lots of things like on many cars....everything from environmental factors to fuel to mechanical....
Fuel with lots of ethanol can make extra moisture when it burns, resulting in more steam....add a high humidity level in the right temperature range, and you have "visible" exhaust...that might be the best case..
More troublesome is mechanical issues....burning or leaking coolant, and having it come out the tailpipe....usually a failed headgasket..corrosion from lack of changes or overheating is the usual causes....
Oil tends to be blueish, coolant whiteish, but with a sweetish smell, overly rich mixture can be slightly black....
Pretty much the for every car ever made....nothing unique on a mini.
My suggestion...since the car has been sitting for "quite some time" I'd bet on the moisture....sitting cars can get piston rings stuck and burn oil till they get unstuck,but again, just a temporary issue from sitting.
One thought....a car that is idled to warm up never really gets hot enough to burn off much of the moisture that collects in the oil and even the exausts....so when you can, go for a drive (check your fluids first). Cars are meant to be driven....
If it is a headgasket, the oil will look like a gross milkshake, and the coolant low....and the exhaust will smell....lack of coolant changes (due every 4 years) or overheating causes this in many cases.

 

I agree, I bet part of this is due to it sitting for a little while. Do you plan to drive it more often or are you still waiting to do the brake job? If you do plan to drive it more, give it a few days of driving to see if the visible exhaust becomes less apparent.