Originally Posted by SkyTeem

Upon gate arrival, you sometimes/often hear the on board announcement "Flight Attendants, all clear," which means the flight is not being met for random drug testing.

It's not so that they can then feel free to light up! It's because some flight attendants "hold it" towards the end of the flight so that when tested upon landing, they can produce a sample. The "all clear" lets them know it's ok to go pee!