The difficulty of building a sensor usually depends on how sensitive it needs to be. In other words, the ratio between the background signal and the small variations in that signal that you want to detect. For example, any cheap microphone can pick up a person's voice, but put that person in a room with other loud noises, and you'll need more sophisticated equipment.
So I think the gravity detection thing would be even worse planet-side. I'm not saying it's not doable, but sonar is already pretty effective at underground/underwater detection.
As for the nekos, I think Wes designed the S series to evade enemy personnel, thus the optical and thermal camo. In specific scenarios, it would probably be easier to neutralize sophisticated sensors through other means.