I agree with dlu. I would check the radiator cap next if the thermostat is known to be good and the car doesn't show that it's overheating on the gauge.
Here's a picture of the functionality associated with the radiator cap.
The cooling system builds up a lot of pressure as it reaches operating temperature, so the radiator cap is designed to relieve this pressure. When a certain pressure is reached, the spring on the cap is compressed and the valve is opened on the radiator cap. This allows for the extra pressure and any coolant that gets forced out with the pressure to move into the coolant reservoir/overflow tank.
If the seal on your radiator cap is bad, the pressure in the system has to go somewhere so it follows the path of least resistance (past the bad seal). This would allow the compressed and hot coolant to overflow out of the normally sealed system.