I can see you did not actually test your project with a single sensor to determine how quickly a person is sensed and how quickly that sensed person goes away. A person's IR temperature is the temperature of the air surrounding that person. When that person moves, the warm air is still there for some period of time until that air mixes with the surrounding air and the IR from the air diminishes. Have you seen television of an IR video of a moving person. There is always a trail of images following the person and they slowly fade away. You have created the same scene with your project.