So what happened? Was it a leak? Nope, no leaks. The ball was outside long enough and the temperature was low enough outside that the air temperature inside the ball dropped quite a bit. When air temperature drops, the particles in the air move around more slowly and bounce against the sides of the ball wall less frequently. This results in a drop in air pressure inside the ball and thus explains why the ball appears to have lost air, as shown above.
Once the ball was brought inside it only took a couple of minutes for the air temperature to increase and for the ball to "re-inflate". No air was added to the ball, but as the air particles inside gained speed, the inside air pressure increased.
Very cool! This just goes to show you that science surrounds you and is easy to find if you just take a moment to stop and look for it.