In order to stucco up the fake brick surface which was on the inner side of the wall, Smiley would have had to seal himself back inside the vault, and so he could not have simply "left the same way he came" (i.e., riding with the other two villains inside of the drilling-sub, back out through the tunnel they'd dug) as Hound describes it. Smiley would instead have had to somehow pass through the vault's ponderous locked door and then also sneak out of the bank without being observed.