Hello Funs,
I would ask for your kind suggestuion and advices. I got the message from the computer, that coolant level low, has to be checked. I refilled it, and the the red sign disappeared. It is possible that the level is going down, or it means that i have a leak somewhere in the cooling / radiator system, and i have to take the car to the service?
The engine never became hot, the temperature was always Ok, in the middle has not warmed up.
Thanks a lot, waiting for your suggestions, I am a new owner, i took about 1000 km into the car, and it has happened... I am a little bit affraid...

Mine (2013 981) did the same thing so I added 50% water 50% coolant per the manual and it's been OK since. That was around 5,000 miles ago. Should add that the owner's manual said to take to the dealer but the dealer said to add coolant first.

There's a leak. It can be anywhere but one of the most common leaks is from the coolant cap. You can replace this yourself. When the engine is dead cold remove the old cap. Clean where the cap screws on to the tank, the threads and the top where the cap must seal. Top up the coolant level with distilled water. Fit the new cap and be sure it is screwed on securely.

Keep an eye on the coolant level. If it goes down the leak is elsewhere. (The cap might have still been leaking, it is just there are two leaks.)

For the leak you should get the car safely in the air and check under the car for leak sign. A very common leak source is the water pump. Another can be the coolant tank. The radiators can leak too, to be sure. If you look in the radiator ducts and spot lots of trash (plant trash and just plain old papers and such) a radiator may have developed a leak from the presence of this trash which holds moisture and leads to corrosion of the radiator (and A/C condenser).

There are other possible leak sources. Any hose, hose fitting, gasket or seal that keeps coolant inside the engine is a leak source, but thankfully leaks from these are relatively rare. Up in the air the tech will check all leak sources.

Just a heads up: This leak should be found and corrected. The leak can let pressure bleed away and it is the pressure in the cooling system that keeps the coolant from boiling as it gets hotter than 212F. (My Boxster's coolant has reached 226F upon occasion.) Lack of sufficient pressure due to a leak can result in the coolant flashing to steam at the hottest spots in the engine and this steam can form a pocket that results in localized overheating of the engine. This overheating is not reflected in the coolant temperature so you won't even know it has happened until the overheating makes its collateral damage known by a possible head gasket leak or a cracked head.

When changing my oil this weekend I noticed that the cap had let some coolant seep through and it left a white residue inside the fill area. I did check the coolant and the level was at the min line so no reason to add. The culprit, the o-ring that sits on the coolant cap had compressed and you could see the exact path the fluid followed. I could see this being a every 3 year maintenance item to keep things properly sealed and as Macster suggested I would look there first before looking anywhere else.

It would be nice if we could simply replace the 0-Ring rather than the entire cap.

For the DIYers here, you may want to track down a coolant pressure tester (auto-zone will actually loan this tool out if you don't want to spend the $100 to buy one), and test both the radiator cap and the coolant system from time to time. If you pressurize the system cold (I'll try to find what pressure the factory recommends but am guessing 15psi would be safe) and leave it that way for five to ten minutes, it should hold its pressure. If it doesn't, then you can start looking for sources of leaks but I am guessing the common culprit at this point would be the cap and o-ring.