What about if they do it in private, but in a way that the information still leaks? For example, say they are in a car or in their own room, but you can, stand

What about if they do it in private, but in a way that the information still leaks? For example, say they are in a car or in their own room, but you can, standing in a public place, watch vibrations of objects in the same space to determine what is being said (I've seen such technology before, and while it doesn't keep voice well and as such you can't identify who said what, you can generally determine most of the words being said).