It could also be one of 2 other things, both of which should be taken care of.
1st, it could be your radiator cap. The radiator cap is a 2-way valve with a spring that is designed to only allow coolant out to the reservoir WHEN THERE IS TOO MUCH PRESSURE. If the spring has weakened, you won't be holding the correct pressure in the system, and you will end up with overheating issues when coolant drops below a certain level. It's a cheap fix and easily sorted, but will save you thousands, or save you getting stranded.
2nd, I have seen this once, and it's a real danger. There are two rubber hoses. One from the radiator, and one overflow outlet. The one from the radiator goes down into the fluid, so when the radiator cools, it draws coolant back into the system... not air.
If you get these two hoses mixed up, then any over-pressure will drop coolant into the bottle, which will then be forced straight up the pipe by the pressure and out the overflow outlet. Also, if the hoses are installed ass-about, then the hose going back to the radiator will not be picking up the coolant from the bottom of the bottle. Instead, when the radiator cools, it will suck in air. Do this too many times and you can blow a head gasket if the car is driven hard or under heavy load.
This is a well-thought-out system. Make sure it's right. Just a couple of low-pressure hose connections could cost you your engine.