thermistor in HC-SR501 PIR
I don't know how you searched on the internet, but the very first document popping up in Google has this:


  Temperature compensation (optional, factory reset): In the summer when the ambient temperature rises to 30 ° C to 32 ° C, the detection distance is slightly shorter, temperature compensation can be used for performance compensation.


The second document has this:


  The HC-SR501 circuit board has solder pads for two additional components. These are usually labeled.
  RT – This is meant for a thermistor or temperature-sensitive resistor.  Adding this allows the HC-SR501 to be used in extreme temperatures, it also increases the accuracy of the detector to some degree.
  RL – This connection is for a Light Dependent Resistor (LDR) or Photoresistor.  By adding this component the HC-SR501 will only operate in darkness, a common application for motion-sensitive lighting systems.