My car seems to be losing coolant, the first time I checked the expansion tank after buying it it seemed empty, so I filled it to the line with a 50/50 Anti-freeze/Water mix. I still seem to keep having to add coolant on a weekly basis, the level drops by a couple of centimeters per week. I can't see any leaks and there are no puddles under the car. The car has never overheated, and there is no white smoke from the exhaust. There is no oil in the coolant or vice versa. The radiator is fairly new and is in good condition. I can smell anti-freeze from the general area of the engine bay when stood outside the car when the engine is running, and also through the interior air vents, so I guess there must be a leak somewhere.