EXTREEEEEME
[It's midnight. A small building has appeared on the lawn, in the middle of the field just outside of the castle, maybe a two-minute walk from the front door, and clearly visible from the windows. It's not very large, either; perhaps the size of a small bungalow. Looking through the glass windows shows a plain white lobby, very minimalist and "modern." There seems to be what looks like a receptionist's desk inside, with a pot of pink flowers. The front door is locked.
It's indestructible, and immune to scans/x-rays/"senses" to see within.
But hey, can't hurt to poke around, right?]