Mostly users find there are quite amount of false alarm from PIR motion detectors even those they have been claimed pet immunity.
PIR is not so miraculous that can distinguish between genuine human and animal, but it does make some assumptions based on the physical difference between animal and human and do some adjustment to the physical capability of the PIR motion detector to make the work more feasible.
Users should understand the design of each individual PIR motion detectors. There is no better way to have your work smooth than having a closed look on the manual provided.
I think users should keep an eyes on three simple things.
1) The height of the detector mounted on wall.
2) The angle of the detector tilted towards the guarded areas.
3) what assumptions have been made to the animal, if you have something out of the cope of their assumption, should choose another model of detector.