PIR Motion sensors are temperature-sensitive. It’s the reason that many of them are intended for indoor use only. Even a passing warm breeze, or cloud cover on the sun, can create enough of a temperature differential to set them off.
This is why the placement matters so much and why one person might have no false alarms at all with a particular model and another one have them every few minutes. A climate like Florida’s, for example, with humidity, breeze, warmth, but variable temperatures during the day, plus flying insects, is famous for setting off motion detectors with temperature false alerts.
Motion detectors also do much worse when they are right on the window ledge because the window itself creates temperature variance due to the lack of insulation in the glass. It will get hotter in the sun, it will get colder as the weather gets cold then wood or brick.
So typically if you did want to put a motion detector outdoors on a sheltered porch, you would keep it away From windows, instead putting it in a very sheltered area against wood or brick. (And avoiding areas of reflection.)
Even indoors, a motion detector near a window often varies in temperature by 10° compared to the rest of the room. So again, we try to avoid windows when placing motion detectors. They’re good places for contact sensors. But not motion sensors.
Air-conditioners and heating systems can also cause problems. Many people get false alarms because they place a motion sensor in a room with an air conditioner in in the winter, not realizing that temperature variation as the air conditioner comes on and off in the spring will set off the motion detector.
Zone detection is a good way to handle this, although even that won’t necessarily work outdoors in Florida since warm breeze might indeed hit all three motion detectors at the same time.
So sometimes it’s just a matter of trying several different placement areas for a few days until you can find an area of stable temperature to create the background against which the introduced motion will be measured.