Even though, that demon possessed that doll, it can move around the house without needing a carrier. It's just like a spirit, a bad one, which can move around without requiring a physical thing. We've seen this in the story told by Esther, mother of Annabelle (Bee).
Esther: She wanted permission to move into the doll so she could be with us forever. We said yes, and that's when it became stronger. Soon, we could actually see her. Only in fleeting glimpses, but it was enough. Our beloved Bee, our darling. Even though I couldn't hold her, it was comforting to have the buzz of activity again.
Here in this story, Annabelle (i.e. the demon) can be seen moving around. That doll was just a temporary house (unless it is a soul) for that demon. Therefore, locking it away wasn't the solution at all. It was attached to doll so hard that it couldn't be separated which is why the doll was locked away and the demon got into hibernation.
It got awakened after sensing the presence of young girls in the house and started to look for the weakest member, which apparently was Janice. Therefore, it started to throw notes around like the time when it asked Mullins to move into the doll.