Humans can be detected by the surface temperature of their bodies. One sensor device that can be used to detect human presence based on temperature is the D6T MEMS thermal sensor: https://www.omron.com/ecb/products/sensor/11/d6t.html.

You can connect this sensor to an Arduino board or Raspberry Pi. It delivers sensor output through the I2C protocol. Here's how it looks:

The FLiR Dev Kit is a thermal imaging sensor from Pure Engineering that can be attached to Arduino and Raspberry Pi boards. The product can be reviewed at http://www.pureengineering.com/projects/lepton and is shown in the next image. You can also find it on SparkFun: https://www.sparkfun.com/products/13233:

Technically, this sensor will a generate thermal imaging photo. If you see red pixels in the photo, it means that area has a high temperature: