(restate) PIR sensors trigger when ambient temperature is shifted enough from stationary or humidity changes in erratic behavior. Humidity can throw temperature as well.
Some types of clothing are invisible to PIR, Rayon, and some Nylon blends leaving only small exposed areas to be potential detection for the PIR, mostly the face, wrist, ankle areas, unless wearing a certain material cover over those exposed areas.... The temperature difference is there trough the material yet not likely to be an adequate enough to result a detection with some PIR sensors. Or adjustments made to reduce false positives.
Many other sensors can false trigger as well. (Options) Multiplexing is costly yet can reduce false triggers. Strain gauge + detection + audible, make for a decent assurance against false spikes as no weight on the strain gauge or plate would cancel a false from the others and so on. Yet having multiple sensors can result in more that can go wrong with the sensors themselves. It's never easy to build any foolproof system.
The RCWL 0516 does show to work based on the method of the transistor oscillator, Colpitts based design. (have used a few)
The module functions like an X band module that has been folded with the output on one side and the receive on the other.
However
The RCWL is an inductive based environment reflection concept.
Inductive objects entering the range alter the return signals causing the oscillating transistors base to fluctuate as the pcb trace is the inductor that completes the oscillators function to operate. It's metaphorically a " pull " generator. The transmit (non component) side transmits each pulse at frequency pulling a small value from the transistor side setting a level, the rest goes beyond sample converse.
As is to why the sensor can perceive inductive objects from the sides at considerable range.
Rain droplets can cause enough reflection value to result in a false output when "close enough" to the RCWL. Have not had an issue with rain yet, no sensors are mounted close to any exterior for possible faults caused by rain or equivalent.
Just note if used that the transmit side is the non component side and the receive is the component side. with a bit of signal tunneling through the pcb. Tho the signal can sometimes transmit through what is behind the board (RCWL logo side) if placed against a wall as an example.