No, that’s not quite how the “motion sensor” technology works. They measure tiny changes in heat as it passes across the sensor. They do report “inactivity” which can be used to trigger off, but of course that gets fooled by the person sitting still.
An alternative, which is discussed in the bedroom project thread I already linked to is to use either a pressure Mat (which, for example, can be placed under the sofa cushion) Which then does detect whether someone is sitting in the chair or not. Some people put these under a mattress, or even under a dog crate.
Another alternative for some use cases which Jody already mentioned upthread is an acceleration sensor on a chair like an office chair, which will catch very small changes in motion.
So again, it comes down to analyzing exactly what the use case is and then determining what combination of devices can best solve it.
At my own house, I often use intentional touchless switches, usually a motion sensor in a box to limit its range. So I catch people walking in with one sensor, but when I want to Turn off the lights I use a different sensor with an intentional hand wave as I get up from the chair or the bed. For example, I have one sensor on the side of the nightstand facing towards the bed. Easy to trigger just by hand position as I get in or out of bed, but doesn’t get accidentally triggered by the dog. 
Again, many different ways to solve the same problems. Just depends on your exact set up as far as what will work best.
And of course now days at our house we use voice as our primary means of light control using the Amazon echo. Works great. But I do still use the touchless switches particularly for late-night use cases where voice might wake up someone else.
Also, the following thread discusses some of the more sophisticated (and more expensive) people counting options, like trip beams at room entrances. (Terry mentioned these at the very beginning of this thread, but I wanted to give the link to the detailed discussion as well) . These are more complex to set up as well as more expensive, so most people aren’t interested in them, but some are. Choice is good. 