We call that "the look." You see it most often when he's been sitting at the top of the stairs for a long time waiting for someone to take him out. It means, "Seriously? My bladder is so full it's pushed all my organs into my throat. You've stepped over me three times. I know you know I'm here." (That face is also a dead giveaway that he's been watching Taxi Driver again.)
See those two whitish spots just below his shoulder blades? I love those spots. Sometimes I scratch him there and whisper, "That's where your wings used to be." Then I imagine Peewee, not as an angel, but bumping around the living room and knocking shit over like a giant bumblebee.