That was a mistake, Never open the radiator cap to check the level. Only check the level in the reservoir.
In a modern car the system is essentially sealed, with the cap mainly there for replacing the coolant.
So you check the level in the res, if its low, add coolant to the res only. If it keeps dropping, you may have a leak.
Also, as stated above, check the level when its cold, so you have consistency with the readings.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i