Anyway, I would very much like to discuss this with the bright people frequenting this forum
build around 30
I've done something similar in the past with visible light phototransistors.Variations in ambient light or reflections from the victim kept retriggering the sensor until the person left, fell asleep or died.
I suspect that the bright people of this forum would find your project distasteful.
... Using a SR04 and coding things to have at least one measurement near by (example 3 feet) within a period of time should work. ...
Have a look here for a similar request and some imaginative solutions:https://forum.arduino.cc/index.php?topic=380567.0
You should be spending your time looking for a way to measure how productive the people are, not how much time they spend at their desks. That is bad management. They could be asleep at the desk!