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. The method comprises the steps of: providing a first propagation property discontinuity at a first distance from a reference position at a top of the tank; generating and transmitting an electromagnetic signal; propagating the transmitted electromagnetic signal towards the product contained in the tank; receiving a reflected electromagnetic signal comprising a plurality of echoes resulting from reflections at propagation property discontinuities encountered by the transmitted electromagnetic signal, including a first reference echo resulting from reflection at the first propagation property discontinuity and a surface echo resulting from reflection at a surface of the product contained in the tank; identifying the surface echo; determining the filling level based on the surface echo; evaluating a first portion of the reflected electromagnetic signal exhibiting a time-of-flight corresponding to the first distance from the reference position; determining, based on the evaluation, whether or not the first reference echo is detectable in the first portion of the reflected electromagnetic signal. If it is determined that the first reference echo is detectable in the first portion of the reflected electromagnetic signal, a first signal indicative of the filling level is provided; and if it is determined that the first reference echo is non-detectable in the first portion of the reflected electromagnetic signal, a second signal different from the first signal is provided.