In my installation one is aimed at a flight of stairs, no way the cat
could be on the stairs and not in range of the detector, yet it does
*not* set it off, but a human anywhere in range does.

That's because the detector has a micro processor to analyse the IR field
and so the detector is "deciding" not to activate when it sees the cat.

What I am saying is that these type of sensors do work but are not fool
proof. If, for instance, your cat gets a fright and charges up the stairs
the detector will "probably" go into alarm. People with a dog anything
bigger than small should also avoid these sensors as the IR field from the
dog could cause an activation as the PIR has to alarm should there be any
possibility of an intruder and a medium to large dog will produce the same
amount of IR energy than a small human (kid).