I noticed on Ryanair that when the captain when to the toilet, a Flight attendant had to get into the cockpit and stay there until the captain had gotten in again, why?
The procedure is there for a couple of reasons. If anything happens to the pilot remaining in the cockpit then the cabin attendant can let the other pilot back in with the minimum of time wastage. Also they can scan the area outside the toilet to make sure no one is loitering with intent and will usually check before opening the door with the remaining pilot who might have access to cameras. Also rules require that at least one pilot remains at the controls at all times. Some aircraft types may require the door to be physically unlocked at the handle which clearly the remaining pilot cannot in reality do properly by themselves whilst flying the aircraft and monitoring the radios. Sometimes the remote door unlocking mechanisms will be u/s as well so the pilot cannot open the door themselves from their cockpit seat so we will need a helping hand.