Proximity sensors for detecting humans as they get somewhat close

• https://www.reddit.com/r/arduino/comments/y8ma8l/best_proximity_sensor_tech_to_detect_a_person/

Infrared camera

Since a PIR sensor is suggested so often, but a PIR doesn't tell you how close the human is, what about an infrared camera that can do facial recognition?

• https://github.com/xyth0rn/ThermalFaceDetection
• https://github.com/avshet/Face-recognition-in-Near-InfraRed-images
• data sets:
  • https://github.com/maxbbraun/thermal-face
  • https://github.com/Nexdata-AI/5438-People-Infrared-Face-Recognition-Data

You're looking for a "full spectrum" camera. RGB-IR cameras do exist, that do multispectral imaging, however, I think that's rarely used for most facial recognition.

How to modify a webcam to be full spectrum: [1]

The facial recognition on an iPhone uses infrared illumination so that it can work in the dark as well.