I have a kid who it sounds like is basically like yours. He doesn't throw tantrums...well pretty much almost never and when he has it's beena symptom of something else. I attribute it mostly to not giving in from the very beginning when he tried it. I'm not a harda_ _, but I was determined not to be trained by a toddler. :) (Okay, it also doesnt' hurt that he's inherited dad's happy-go-lucky personality!)
He'd also been very predictable in his sleep patterns. Until age 2, he'd go down without a fight and stay down all night. At 2, staying in bed was a big issue and he'd come into our room (we had switched him to a toddler bed around that time which I think had attributed to the problem) crying he was afraid of sharks and dragons...go figure since there was nothing he'd watch that had either and there weren't any books we could think of that he would have gotten it from.
Long story short, what stopped the behavior was leaving a light on in his room. Although now at age 5, he won't let us turn it off!!!! But that might help.
I do agree with some of the comments about going in to her and see what it is she wants and what's she's afraid of. I honestly think if she IS afraid of soemthing, whether it's with words or gestures she will be able to tell you. Somehow. If she doesn't or can't, I'd say try the light. It might be that little bit of glowing comfort that she's looking for. But again, that's just my opinion for what it's worth.