No way that those triggers are anything other than the vehicles. They are all in the exact same place at the start of the clip. What are the odds of that, lol.
Can’t speak for why they are doing it, if they are masked off, but no doubt in my mind that three vehicles, in precisely the same starting position, are the things causing it. It is inconceivable that you got three clips triggered like that, by the bush.
When I said I can’t speak for why they are doing it, I’m going to take a stab.
Motion is not masked at the PIR sensor, can’t be, it is masked by image comparison in the camera, or so I have been informed. Every time motion is detected by the PIR it has to do this comparison, and masking cannot change this since they are separate.
PIR ‘sees’ vehicle and image comparison is performed. Vehicle would be, and probably is, ignored, it is in a masked out area. Other areas, that are not masked out are checked for motion, again by image comparison. It sees the bush, sees some motion in the shadow, and decides that it needs to capture it. Next thing you have a clip.
Wouldn’t be surprised at least.