Well well here are the basic tips that i can provide you in my own experience.. I maybe too late but just wanna add some helpful tips.
1 - Leaky Radiator Cap
If you have a leaky or a weak radiator cap, you may experience loss of coolant from the overflow tube every time the coolant heats up. The radiator is a pressurized system so if there is a loss of pressure from the cap not fitting property or if the cap is the incorrect cap for the radiator, the pressure will be lost and a leak could occur. This can be diagnosed with a pressure test of the radiator cap. Refer to the car’s manual for the proper cap and pressure.
2 - Internal Leak
Although this might not be a leak as we tend to think of a traditional leak, for example the puddle of coolant under the car, an internal leak would be noticeable when the coolant level does not stay constant and you find that you need to constantly refill the tank or your engine overheats. An internal leak could be caused by a leak in the head or block or perhaps a leaky head gasket which lets the coolant escape its system.
3 - External Leak
An external leak is the type of leak which is most easily diagnosed. More often than not, when you have an external leak, the coolant escapes and your car quickly overheats. Most likely you can easily spot where the leak is coming from by simply looking at it: a split or broken hose or a hole in the radiator are two good examples. Often times leaks in the water pump, heater core or engine freeze plugs can allow coolant to escape. These too are easy to diagnose by a visual inspection. I hope it help others to avoid going to your dealer just because of your simple vw passat coolant