[Jaydee]

Any engine that warms up will create some pressure as the coolant expands, initially. If you release this pressure, then refit the cap, there will be no more pressure while the engine is warm, obviously if it start to overheat, it will then build up pressure to keep it from boiling.
If you don't get that initial pressure, it might be the cap leaking. This intial pressure can be high enough to lift the spring in the cap and flow to the overflow tank, then when it cools overnight, it sucks it back into the engine.
Hope this helps.
JD