I know I'm going to regret wading back into Umineko theories but...
In episode 2, I have good reason to believe that Rosa shot one of the people found in Natsuhi's bedroom and locked the door behind her.
Look at the bodies, do you think any of those are bullet wounds? Keep in mind that the room was searched and that no guns were found in it.
Also there's this:
As you can see here Rosa put the keys in Maria's handbag. However when she later produced the keys, there is no indication that they were removed from Maria's handbag. Maria doesn't let people take stuff from her handbag without making some noise.