An air pocket in the system does not develop by itself from nowhere, and anyway would be a temporary situation that would clear by itself by natural means if overflow tank level is maintained; so if you have not had any work done on car like changing coolant pump, flushing coolant and replenish, changing radiator, then the air is taken from somewhere else. Head gasket failure is common and does not give unescapable signs from exhaust pipe - after all water is a natural by product of combustion and there is already plenty of water that goes out the exhaust pipe. In fact, more than 3 quarts of water are exhausted as vapour (except when exhaust pipe is cold, then it condenses) each time you burn 1 gallon of fuel - so a few drops of coolant do not change much.
One other now frequent failure is the intake gasket - now because use of dissimilar materials like cast iron and aluminum is rather recent and aluminum has 3 times the expansion rate of cast iron, so gaskets are constantly rubbed on from both sides - . Intake manifold is most of the time under a vacuum when the engine functions and this vacuum is able to siphon water by a tiny leak that would have developped, and then air enters the cooling system very easily by the radiator cap (that has a one way valve TOWARDS the cooling side that opens at almost 0 pressure differential) as soon as overflow tank is empty. SInce this leak is exploited by vacuum, then at rest, coolant will not invade the combustion chambers, so no sign in oil. IN fact, IF leak develops also to the atmosphere side, some coolant that may leak to the manifold at rest will leak and be seen on and outside of engine. And since exhaust has no access to the intake manifold and to that leak, there are no exhaust bubbles from the radiator.
In fact, one key question here: do you have to add coolant from time to time????? It seems you do since the "low level light" comes on.....so coolant is going somewhere......and air is replacing it.