[kaiser]

An air lock is a pocket of air, that is trapped (usually under a closed thermostat or in a heater or a high point of the cooling system) when filling the cooling system. It will be moved as soon as the car gets up to temperature, the thermostat opens or the heater valve (where applicable) is opened and the water circulates. This is the reason why a top up (or two) is required after filling the system.
As almost all (if not all!) coolant systems are self-bleeding, such an air lock will show up as missing water at the filling point, radiator or expansion tank.

A lack of coolant, subsequent to a full cooling system, can only happen if you lose water, and air is drawn in to replace the lost water. This is not an air-lock. It happens gradually as water is lost and no air is trapped.
This is a loss of water, to the point that the water pump no longer pumps, or the radiator no longer works as intended. In this case, no amount of bleeding will cure the problem, as the loss will continue until found and addressed.

There is a big difference. One is technique and/or care, the other is a fault.