There's nothing else to be released, because when microwave production is stopped the waves cease to exist.
From the FDA's FAQ about microwave safety:
systems that stop the production of microwaves the moment the latch is released or the door opened. In addition, a monitoring system stops oven operation in case one or both of the interlock systems fail. The noise that many ovens continue to make after the door is open is usually the fan. The noise does not mean that microwaves are being produced. There is no residual radiation remaining after microwave production has stopped. In this regard a microwave oven is much like an electric light that stops glowing when it is turned off.
Provided that your microwave meets regulations regarding the double locking mechanism, more radiation is leaked when the door is closed and the microwave is on than when the door is opened and the microwave is off.