Indeed, you would need a rootkit in each target, logging and transmitting the keys. Something like a proprietary instant messaging client or keyboard application.
Remote sensing in this case is listening to the individual sound a key makes when pressed by the victim, and wear on the keys means that frequently used keys (such as 'e' or the space bar) would make a slightly different sound than the other keys, as does the hand/finger position used to press a key.
But you don't need that, you'd only get too much information you don't need. You need to intercept suspect network traffic only, otherwise you're most likely to fail to find anything useful at all.
Quantity has a quality all of it's own. Wiretaps against a specific individuals is a lot different than dragnets covering an entire nation.