The message
from John Rumm contains these words:

IME, they still stop triggering unless the warm body is moving (at least
some of the time)

Quite. If you look at a PIR sensor you'll see there's a moulded plastic
fresnel lens that's split into many patches. The idea is that the sensor
only sees strips of the coverage area, only those parts focussed by the
lens. The bits in between aren't visible to the sensor. The detector
works by seeing a sudden change as you walk from a bit that's covered to
a bit that isn't - or vice versa.

--
Skipweasel
Pay no attention to that man behind the curtain.