worn out head gasket ? I just had the same problem. One day after a drive I turn off the engine and water just started to poor out from the overflow, I was in a hurry so I came back and checked the water level both in the overflow and radiator all was good and drove back home and seems like noting happened.
Then a couple of weeks later the temp started to rise higher than usual so I parked with engine on and checked water level both radiator and overflow, it was still ok but this time the temp was higher like 3/4 of the temp gauge and there seems to be 'air' in the water system from looking into the radiator causing small bubbles all the time and slowly pushing water out (radiator cap off). Rev the engine to see if the water was still moving or flowing and it was. Changed a new thermostat, problem still there. So had no choice but to open her up and found that the head gasket was starting to leak and crack. So in the process of changing it now. Hope that it works !
Sorry for the long post
tried to shorten it best i could. Or another thing to look at is maybe a clogged up radiator or your radiator fans.
best of luck.