[Heiko]

Could be just a hose... I wouldn't panic yet, but I wouldn't drive it unless you know the coolant will stay in the system. If you have a place you can raise the car and get underneath to locate where the water is coming from it would be best, otherwise have it towed somewhere. If the tank was leaking, you'd have coolant under the trunk mat, if the coolant cap or pressure release was leaking it would be coming out in behind right rear wheel (at the drain hose).... exact middle of the car are just the hoses which of course are covered by plastic protective panels, so if there is a leak, the water will find its way to either end of the panels; near the front of the engine is the water pump and thermostat housing and of course your hose connections... so there could be lots of places.