Point of order. Many PIRs have just one pixel. A variation of heat from what it was half a second ago is taken as movement.
Fair point. I was wondering while I was writing that whether a single pixel one would work. Wonkipedia suggests two pixels ones have fewer false triggers.
The single pixel ones can desperately sensitive to any sort of noise. In a typical sensor the lens would typically give 5 bands of sensitive and 5 of insensitive in an arc of 120°. To pick up a person moving, the sensor has to detect stuff that changes in the sort of timescales of a person moving around in a room, so taking a second or two to move in or out of a sensitive area.
That leaves the electronics horribly sensitive to supply voltage variation, or flashing lights etc, and remembering the value from a second ago needs a big, and therefore leaky, capacitor. Doing it digitally is probably too expensive for the dynamic range needed.
I could see that having two pixels would reduce that tendency lots, so when the cooker turns on, the supply voltage dips, and both pixels show a dip, the electronics ignores it.
I think there would still have to be some memory of recent levels of IR, or a hot pipe in the view of the sensor would set it off.