PIR motion detector or Emitter receiver pair or?
just to officially post an answer to this question, reposting what @SteveG said above in the comments, a transmitter receiver pair is the most suitable option.

A PIR is  deemed unsuitable as it 


  detects a difference in temperature between a moving object and the background


and as such is unsuitable to detect an object that is likely to have a similar temperature to the ambient temperature