Rex's Reach
Rex, thinking fast, tears a small strip off his shirt and stuffs it into the latch of cell door, quietly shutting it once more, and holding it shut. He stops the lullaby, because it no longer serves a purpose.
He wasn't sure if he needed to keep the cell door shut, or if it would stay open, but he wasn't taking any chances. So, he held the door shut for now, and waited to see if the guard would wake up or not.