While drilling, there will be a certain amount of the gas in cuttings entering into drilling fluid when we drill through porous formations that contain gas. The gas showing on the surface due to drilling through formations is called “Drilled Gas”.
When gas from the cutting comes into drilling fluid, it will expand as it is circulated out of hole, hence, you will see the gas from the monitors at the flow line. Even though we have overbalance hydrostatic pressure exerted by mud column, gas showing on the surface by this mechanism always happens. You cannot rise mud weight up to make it disappear.