Why coolant reservoir is empty?
 

I have Cooper S 2009 (R56). I noticed that the coolant reservoir is empty. Even after I refilled it it got empty after a few days. I took it to the dealer and they told me that I need to replace the thermostat (the diagnosis was for free). I didn't have any engine check. The interesting thing is that after I returned from the dealer the reservoir didn't empty for at least a week. After a week the level of the liquid decreased to min mark and stays there. It's been already 3 weeks and the level didn't change. Can you suggest what could be the problem? If I have the leak why the water stopped decreasing?

Did they replace the thermostat housing ? That would fix the leak and then when refilled it would stay the same once bled.

@ECSTuning, no they didn't. In addition, it's been a month already and the level of the coolant is STILL on MIN mark (it didn't change). I stopped the car after driving an hour (the engine is still on) and I noticed that the coolant doesn't boil. Shouldn't it boil at least a little bit when the engine works? I recorded the coolant and this is the link to a video. Is it fine? I also noticed that sometimes, rarely, I hear a fan starts to work vary noisy and then stops.

Hello, did you see the other post on NAM ?