I was in line at a fast food place last week, maybe two weeks ago, started over heating. Turned it off let it cool, drove home. Checked the reservoir, empty. Filled it to the line.

Hadn't had any problems but the fan has been staying on. Tonight it did it again. I replaced the thermostat about a year, year and a half ago.

Any ideas on the culprit. My associate cooper is a 2005 S R53 BTW.

 

Check the hoses for any leaks, especially where the hose meets the reservoir.
Also check the reservoir itself for any separation along the seam line.

 

Overheating might be expected with very low coolant..... So keep an eye on the level...but it might be a contributing factor in this case....
Did you hear the radiator cooling fan running?
On pre jan2005 production cars, the PS cooling fan and radiator cooling fan are wired so if the tiny PS cooling fan jams, it can blow the fuse that powers the radiator fan relay...stopping the radiator fan too...so the car can overheat when not moving.

 

If it was low on coolant, it will cause overheating and it's most likely leaking somewhere. Check the hoses, overflow tank and the area on the driver side of the s/c where the water pump and thermostat are. Overflow tank are known to split in the middle and a leaky thermostat housing happened to a couple of people. If it doesn't leak anywhere but the coolant keep disapearing, then you might have a bigger problem.

If there's a leak, fix it, then you'll see if it solves the problem. If it doesn't, it can be a faulty thermostat or like ZippyNH said, a blown fuse preventing the fan from working.

 

Search on the forum for low speed fan problems. There is a low and high speed fan setting controlled by a resistor that is known to go bad.

Anyway, The high speed fan only runs when A/C is on or when the temp gets to a certain degree.

Therefore if you were sitting, idling, then the car was not getting cooled because the low speed fan was not spinning. And the low speed fan is much quieter so you probably don't hear it anyway.

Once you are moving again the car can cool down.

So if you are not finding a leak. You may just have a bad fan resistor.