Since the expansion bottle in the IS is not pressurized, when re-filling coolant after a drain, the re-fill happens through the expansion bottle. Has the bottle got direct flow into the coolant system? If so wouldnt that cause pressure in the expansion bottle?

 

It works much like a pressure cooker. As the coolant heats up, it expands and pressure builds, but a spring-loaded plunger in the radiator cap maintains pressure. When the pressure is too high, the plunger will move and allow coolant to escape the main system and travel to an overflow tank.

 

What Sceanzo says is true, but the other half of the story is that when the coolant in the radiator later cools and contracts, it creates a vacuum that siphons some of the coolant out of the bottle and back into the radiator.