It might only time will tell really
I personally think the keyhole has to do with either one of three things
1. One houseguest will get a key to the BBAUS house or even BBUSA house.
2. The key will unlock a luxury type of room.
3. The key will be like when Grace entered the house on her 21st B-Day one hg in the main house or the house next door will get into another house.
BTW there are 50 other topics on this.