A method of monitoring operation of a radar level gauge system installed at a tank and arranged to determine a filling level of a product contained in the tank, comprising the steps of: providing a probe extending into the product contained in the tank; providing, along said probe, a first propagation property discontinuity at a first distance from a reference position at a top of said tank, said first distance being selected to define an overfill detection zone above a normal filling level range for the tank; generating and transmitting an electromagnetic signal; guiding said transmitted electromagnetic signal along the probe towards and into said product contained in the tank; receiving a reflected electromagnetic signal comprising a plurality of echoes resulting from reflections at propagation property discontinuities encountered by said transmitted electromagnetic signal, including a first reference echo resulting from reflection at said first propagation property discontinuity and a surface echo resulting from reflection at a surface of said product contained in the tank; evaluating a first portion of the reflected electromagnetic signal exhibiting a time-of-flight corresponding to said first distance from said reference position; determining, based on said evaluation, whether or not said first reference echo is detectable in said first portion of the reflected electromagnetic signal; determining, based on said reflected electromagnetic signal, if said surface echo can be identified; if it is determined that said surface echo cannot be identified and that said first reference echo is detectable in said first portion of the reflected electromagnetic signal, providing a first signal indicating that said filling level is below said overfill detection zone; and if it is determined that said surface echo cannot be identified and that said first reference echo is non-detectable in said first portion of the reflected electromagnetic signal, providing a second signal different from the first signal indicating that said filling level is in said overfill detection zone. providing a probe extending into the product contained in the tank; providing, along said probe, a first propagation property discontinuity at a first distance from a reference position at a top of said tank, said first distance being selected to define an overfill detection zone above a normal filling level range for the tank; generating and transmitting an electromagnetic signal; guiding said transmitted electromagnetic signal along the probe towards and into said product contained in the tank; receiving a reflected electromagnetic signal comprising a plurality of echoes resulting from reflections at propagation property discontinuities encountered by said transmitted electromagnetic signal, including a first reference echo resulting from reflection at said first propagation property discontinuity and a surface echo resulting from reflection at a surface of said product contained in the tank; evaluating a first portion of the reflected electromagnetic signal exhibiting a time-of-flight corresponding to said first distance from said reference position; determining, based on said evaluation, whether or not said first reference echo is detectable in said first portion of the reflected electromagnetic signal; determining, based on said reflected electromagnetic signal, if said surface echo can be identified; if it is determined that said surface echo cannot be identified and that said first reference echo is detectable in said first portion of the reflected electromagnetic signal, providing a first signal indicating that said filling level is below said overfill detection zone; and if it is determined that said surface echo cannot be identified and that said first reference echo is non-detectable in said first portion of the reflected electromagnetic signal, providing a second signal different from the first signal indicating that said filling level is in said overfill detection zone.