Someone is jumping out of a window. A crowd of photographers is there trying to take pictures. A person gets in the way of the picture one of them is trying to take, and the photographer yells "13 and 8!" That's the yell that they customarily use when someone tries to get in the way of a picture with the hope of getting to be in it if it's in the paper the next day. This warning lets all the photographers around be on their guard.
The photo comes out with a perfect picture of the person jumping out of the window... and with no sign of the person who got in front of it. The person was there when the picture was taken, but no one else saw him and he doesn't show up in photos. This happens again several times, and people think the photographer is just trying to distract everyone else so that he can get a better picture. He knows someone's there, but he can't prove it.
"Thirteen and eight!"