You know why? Because you're breathing CO2 to them
If the scientist that hurt the plants walked into the room, the plants would react straight away - and go into distress mode.
but if the scientist that was nice to the plants would walk into the room, the plants would react as well(not go into distress, obviously), they would even react when he had sex miles away (later confirmed).
I don't think plant can sense people...Plus, its impossible to hook up a plant to a polygraph machine...They don't have brains or hearts!