Originally Posted by
FlyingHoustonian
There is a metal cover that can lifted up to slide the door from locked to unlocked on the outside. Often the crew will keep it locked and latched closed so the light stays on. Anyone can lift the metal flap and use a finger tip or pen to slide it unlocked but I suspect the crew would not be thrilled, so normally one has to ask and they will do it.
I would not suggest anyone to unlock it from the outside. More often than not, the lavatories are indeed occupied. I have seen over anxious frequent flyers did this without knocking on the door first while someone was in there. The offender got reported to the flight crews. I was also on the receiving end once, and luckily I was done with my job and was washing my hands when it happened.
When the lavatory light stays on longer than expected, just ask the FA rather than playing the role of a lavatory vigilante.