This concern is usually caused by the air bags dry rotting in the folds at the bottom of the air bag,that is why you can not see it.What happens is where the bag folds at the very bottom over time they will dry rot and crack when you go over a bump that allows the suspension to make a full travel and this then exposes the cracks and the front end drops down. The reason the light comes on shortly after start up is because this system has a timer on it for the compressor to run and get the vehicle to ride height,if the computer sees that it is taking to long to pump up it will set the light. What I usually do to recreate or find the problem is I find a ramp that I can drive the vehicle off of,we have a large one out of our back door at the shop that I drive off of the side of that lets the suspension go thru a full travel(shock fully extended) then come back to level ground,then I immediately get out of the vehicle and walk to the front and listen for an air leak on one of the front bags. Another way is to remove the bag and then remove the air solenoid and then pump the bag up with compressed air and check for a leak.