I've noticed on some flights where I've paid attention that the FA's lock the lav at pushback from the outside (lift the little flap on the door and push the bar to lock it) then will unlock the door only after the seatbelt sign goes off. Or if they themselves need to use it (which is usually the case). Once, the seat belt sign went off and none of the FA's were near the lav, so I went up and unlocked it myself. (Warning - only do this if you've been paying careful attention to ensure no one is already in there. 'nuff said....)