Does she have access to water when your door is open to your room? Because most likely she's going downstairs and drinking water, which is why she's not having accidents when she's locked in the room (could even be from the toilet). If this is the case, simply remove any water bowls overnight, keep toilet lids shut, and she should be fine.
I always completely cut my dogs off from water overnight. Otherwise they will get drinks in the middle of the night and end up waking me up to go outside.
Definitely doesn't sound like she's marking. She is house trained and just can't control her bladder at the time, so she's peeing in the same spot since she can't go outside.
If she doesn't have access to water, perhaps the fact that she's allowed to move around and not simply lay there and sleep is making her have to go. She might be walking around the house while you're asleep, and walking around makes a dog have to pee more, pretty much same with humans.