Acoustic waves in planetary atmospheres couple into the solid surface, producing ground displacements that can be measured using seismometers. On November 26 2018, the InSight mission successfully landed on Mars. Its objectives include studying Mars' interior using the seismometer SEIS (Seismic Experiment for Interior Structures) and the atmosphere through the weather station APSS (Auxiliary Payload Sensor Suite). Because InSight is the first mission capable of studying infrasound on Mars, we investigate the signature of infrasound both in terms of air pressure and ground velocities. Using numerical simulations, we characterize (1) the acoustic propagation pattern in Martian dusk, and (2) the mechanical atmosphere-to-ground coupling under acoustic waves. Then, using SEIS data, we demonstrate that two low-frequency monotone events (S0133a and S0189a) are in fact infrasound trapped in the atmospheric nocturnal surface waveguide. We base our demonstration on the following facts. (1) Seismic signals rarely produce, at a given station, a single frequency varying from one event to the other. (2) No clear seismic phases have been identified for such events. (3) The observed SEIS signals present the characteristics expected for trapped infrasound observed through their compliance effects (specific frequency response, more energy on the vertical component, ±90 • phase shift between vertical and horizontal components, no detection on pressure sensor at these low amplitude levels). Our simulations of the nocturnal waveguide's response is however subject to uncertainties because 1) it relies on the sol-to-sol variability of the atmosphere, and 2) sub-surface properties are not properly known at this time.