What could cause a antifreeze leak when the car is turned off on a 1999 Cadillac Catera?

Check it when it is cold. If it smokes white smoke when you start it, you could have head gaskets leaking.

the pressure is still existant in the radiator..

Usually when the car leaks water when it is off it is because the water pump is weeping.... Starting to go out, you need to replace it

I have had a lot of experience with this one. The design of heater control valve is flawed. The valve snaps together and after a while the pressure in the coolant system will push the top up just enough to leak. As the inner spring gets weaker it leaks more. The replacement valve cost me less than $100 at Advance Auto and took about 2 hours to replace. The valve is located against the firewall on the passenger side of the car.