Marciokoko:
...even with all the sensitivity down to its lowest value via the onboard pot of the PIR sensor...
That is not a sensitivity pot, but a 'delayed response' adjustment, to reduce false triggering.
Modding the fresnel lens (plastic dome) could be an easy way to reduce viewing angle for the sensor.
Try a round piece of paper inside the dome. With or without a hole in the middle.
Bathroom sensors most likely don't use PIR sensors.
Leo..