Mystery door

We are full on with the renovation of what will be our Regency suite in the style of the 1700's. We were stripping off some old plaster in what will be a walk in wardrobe when we found something puzzling. As you will see in the photo when we exposed the stonework there is a clear vertical line of blocks, suggesting that was the edge of an old door. Normally this is replicated on the other side of the wall. However not this wall. There is no sign of this doorway on the other side of the stone wall. A mystery!