Liquid-presence sensor for a container, said sensor comprising: - a first sensing element, suitable for being placed outside the container, and having a first electrical circuit comprising a primary coil, - a second sensing element having a second electrical circuit with a secondary coil and with means that are able to sense the presence of a liquid in the container, - a magnetic coupling between the primary coil and secondary coil of the first electrical circuit and the second electrical circuit, for transmitting to the first electrical circuit liquid-presence signals emitted by the second electrical circuit, - the second electrical circuit being a passive secondary oscillating circuit, comprising the secondary coil and a capacitor, with a variable-impedance element, the impedance of which varies depending on whether the liquid is present or absent, and the impedance variation of which modifies the oscillation properties of the passive secondary oscillating circuit, characterized in that: - the second sensing element is suitable for being placed inside the container, - the magnetic coupling between the primary coil and secondary coil of the first sensing element and second sensing element is without a continuous or closed magnetic circuit containing ferromagnetic material, - the first electrical circuit comprises an exciting electrical circuit able to generate in the primary coil exciting voltage pulses, and an analysing electrical circuit, - during an analysing sequence, the analysing electrical circuit scrutinizes the resulting electrical signals that appear on the primary coil following the application of an exciting voltage pulse, compares parameters of these resulting electrical signals to reference parameters, and generates an output signal dependent on the result of this comparison. - a first sensing element, suitable for being placed outside the container, and having a first electrical circuit comprising a primary coil, - a second sensing element having a second electrical circuit with a secondary coil and with means that are able to sense the presence of a liquid in the container, - a magnetic coupling between the primary coil and secondary coil of the first electrical circuit and the second electrical circuit, for transmitting to the first electrical circuit liquid-presence signals emitted by the second electrical circuit, - the second electrical circuit being a passive secondary oscillating circuit, comprising the secondary coil and a capacitor, with a variable-impedance element, the impedance of which varies depending on whether the liquid is present or absent, and the impedance variation of which modifies the oscillation properties of the passive secondary oscillating circuit, - the second sensing element is suitable for being placed inside the container, - the magnetic coupling between the primary coil and secondary coil of the first sensing element and second sensing element is without a continuous or closed magnetic circuit containing ferromagnetic material, - the first electrical circuit comprises an exciting electrical circuit able to generate in the primary coil exciting voltage pulses, and an analysing electrical circuit, - during an analysing sequence, the analysing electrical circuit scrutinizes the resulting electrical signals that appear on the primary coil following the application of an exciting voltage pulse, compares parameters of these resulting electrical signals to reference parameters, and generates an output signal dependent on the result of this comparison.