Both the solar occultation (SO), and the limb nadir and occultation (LNO) channel work in the infrared, while the third channel works in the UV-visible range. The SO channel measures when it points toward the sunset or sunrise when the orbiter moves toward or away from the dark side of Mars respectively, while the LNO channel is used when it looks directly at the sunlight reflected from the surface and the atmosphere of Mars.
The NOMAD spectrometer and the Curiosity rover have recorded very different amounts of methane in the atmosphere of Mars.
This article in Geophysical Research Letters (https://doi.org/10.1029/2019GL083800)
explains it as follows:
In this paper we describe a framework which explains both measurements by suggesting that a small amount of methane seeps out of the ground constantly. During the day, this small amount of methane is rapidly mixed and diluted by vigorous convection, leading to low overall levels within the atmosphere. During the night, convection lessens, allowing methane to build up near the surface.
Now when during the night that Curiosity observed a transient methane plume an orbiting mirror could have reflected sunlight to the surface near the rover, could the NOMAD spectrometer have detected that plume ?
Could it even have made the detection if Curiosity had been able to shine an intense beam of light on the methane plume ?
More precisely, how intense the light and how wide the beam of light would have to be for the NOMAD spectrometer to detect such a methane plume ?