Never add water until the car cools down.
I would remove the bleeder screw next to the fill cap, fill it up until liquid comes out the bleed screw, (key on, heater full hot setting) and then watch for leaks over the next few days. On an 11 year old car, the first two suspects would be the expansion reservoir (where you add the water) may be brittle and cracked, or the O-ring on the water temperature sensor on the lower radiator hose. That will leak only when the car is hot and there is pressure.
If you have over 150k miles, the water pump may be leaking. That is tough to see until you get all the stuff removed from the front of the engine. By the time you get that much work done, you will want to replace the water pump as preventative maintenance even if it turns out to be still OK.