I have a 2010 mini cooper and have noticed the coolant levels being low.
i have checked on all possibilities including "thermostat, head gasket, and expansion tank!"
My coolant keeps running low after filling, I've noticed no leaks but i have seen coolant on my manifold but no leaks found around any hoses.
When the car is parked and off the coolant will sit in the tank and not leak, but while I'm driving it seems to burn through all my coolant.
I'm not noticing any smoke, water in oil, oil in coolant.
The car is overheating at times but not to bad!
i have been filling up my tank pass the max line and realized that may result in coolant loss and spilling out.
I don't know if thats where my coolant loss is happening from.
I don't think its the thermostat cause my engine does not got overly hot!
and i don't believe it is the head gasket due to no smoke and no oil/water mixup!
ill take any info given!

 

Try a compression or leak-down test. My guess is you have a bad head gasket, if not a cracked head or block.

 

check by the water pump, perhaps a leaky gasket there. if automatic tranny, check the hoses going into the ATF cooler. lastly, follow your nose. search for the SMELL of coolant.
finally, while the car is running, pop the hood and listen for hissing or look for that steam. check around the turbo too.

 

Carefully check the thermostat housing. I bought my 2009 overheated. The owner could not find the leak. The housing cracked, leaking enough to evaporate off and not drip. It eventually burst, overhearing the car. When I removed it, tell-tale signs of leaks and dried up residue at the crack.

Also check the pipe connecting the water pump to thermostat housing. I had to replace it also. Hardly any plastic material left.