Most all of my LD's have been spontaneous like that. It just snaps, and I really don't even have to do a reality check because it's so obvious. I've only ever recognized a dream sign once to gain lucidity, and had one false awakening where I gained lucidity. Other than that, all others have been very spontaneous just like you described.

Like Puffin said, the increase in awareness is what will cause most of them for you. If you aren't aware, your dream signs will basically be slapping you in the face, like they do to me, and I don't even notice. Those are the times that you are kicking yourself when you wake for not noticing them in the dream. My brain seems to like to mess with me, cause I'll have such obvious dream signs...multiple dream signs...they might as well be yelling at me that I'm dreaming, but I go along with the dream as if they weren't even there.