For air to leak out there has to be a leak. It sounds like a duh statement but many people believe that things just lose air over time. Objects made to hold air are made to not leak and if they do there is something to repair. Tires, wheels and air shocks fall into the category of holding air and should not leak if in good condition. Tires according to Goodyear will lose 1 pound of air pressure for every 10 degrees the temp drops likewise, they will gain the same as temps rise. I would think that the air shock system would be more susceptible to temperature changes. I could be wrong about that since there is a very low volume of air to be effected.
As someone else suggested use soapy water to check the airlines and the shock. To little soap in the water and it will not bubble when making the soapy water mix 10 times the soap you think you need and it will bubble the way it should.