Well, since Eva didn't answer my questions..... Guess I just have to shoot my theory.
First off, we know that they entered through a gated entrance, so not many people could have entered easily. Since the culprit most probably would have to know the owner to even enter the compound, she could have came to the house before the cousins have via key from the owner or from the furnace.. So, before the cousins and our detective even entered the house, she stuck something on the area where the bolt would connect to the door. Then, at night, she decided to enter the house via methods mentioned. Now, since the door's locking mechanism was prevented, she could enter through all of the doors with ease(Since there was the 'perception' that the doors were securely locked, no real confirmation). After killing each cousin, one by one, she would close the door from the outside, after which, she remove the object dislodged at the area of the lock. This means that the rooms were not perfectly sealed in the beginning, therefore there was no 'seal' to be broken with.
EDIT: Nvm, didn't see the Knox's truth .-.