In summer is more distilled water than anti-freezer, is evaporating...in winter put some anti-freezer in order to complete until normal level...Do not exaggerate with anti-freezer, if you have a measurer for anti-freezer, use it...remember that water is cooling engine more fast that antifreeze agent...Better to prepare the car for winter in a service, to ensure that coolant is correctly calibrated for winter period.
George is correct. Before the winter starts make sure that your coolant is of sufficient anti-freeze strength to protect you in your country. Some of you have very cold winters.. If you don't have access to an hydrometer get your garage to check. Believe me its much cheaper than repairingproblems caused by frost damage. 2008's have not been around long enough for coolant problems. If your vehicle has lost coolant you have a problem and this should be properly checked. There are many reasons for this. Some simple, some nasty but as your vehicle is still under warranty you should take action with your dealer.
Is not a sealed system. If was, would explode You have the expansion bottle, wherefrom you put extra anti-freezer, have a pressure relief valve cover that allow the accumulated vapors to exit.
The more distilled water you have, the better the cooling of engine...but...in winter you must add anti-freezer, in order to not make ice and crack the radiator...the water is making steam...so...you don't necessary loose the coolant but just evaporate some water.
This is also why after winter season, you must go to service and prepare it for summer, as the anti-freezer is not such high conductible as water, and will not cool the engine properly.
Hope this help you make an idea regarding coolant and its function.
Coolant in a modern engine is a permanent feature and no modern engine is designed to run at any time on pure water; soft, hard, distilled etc.
The coolant has an anti cavitation (erosion) effect on particularly the pump impeller and has a superior 'wetting' effect, important in heat transfer as well as an important anti corrosive intent.
A sealed system with correct coolant, is designed to raise pressure to a controlled extent (with emergency blowoff ability) but evaporation simply shouldn't happen as with cooling the pressure simple returns to 1 bar from 2 for example.
It won't "blow up" as the pressure raises the boiling point and is designed that way specifically for that reason.
I've been working on all my own vehicles for 50 + years and have rarely added water to a modern properly maintained engine that does not have an existing cooling problem.
My wife has a 10 year old Ford Ka and I have had four cars during that time including a Picasso and a 3008 Peugeot as well as the current 2008 and have never needed to top up or otherwise touch the coolant in any of these vehicles. I refer to my earlier post, if coolant is being lost there is a reason for this which should be investigated. Modern systems do not lose water through evaporation.