Roomba LED Long Exposure • r/pics

Not OP, but I work on alarm systems and getting proper coverage is a big part of installation. Passive InfraRed (PIR) sensors are based on emitted or reflected infrared energy as you suggested. This means they can be defeated by an object which is not very reflective and is at room temperature, but our Roomba here probably is warmer than the room since it has a running motor and discharging battery. PIRs have a limited field of view, which is often adjustable. Think of it like a camera, you can point it at the wall instead of at the floor and you'll see people but not small pets.