I think I've found a large part of the answer and I'm posting it for anyone else running into the same problem of spontaneous firing of the PIR output. The PIR enclosed in a black plastic project box produced several "events" until I turned off the halogen and fluorescent desk lights close by. Now, there was only one signal appearing within an hour of inactivity, which is OK for my purposes. Turning the lights back on one at a time brought back the "events" equally; that is, it didn't matter which light was turned on first. Once the "events" started, shutting the lamps off again was not so effective in the short run, but shutting off the surge protector powering the lamps was. I haven't tried shut-off with the other 3 PIRs, but I'm sure they will show the same. I'll experiment with a "sacrifice" PIR to see if a jumper on its PCB might solve the problem. In any case, my project involves all night stints with the lights off.
The other locations I was talking about were inside the house, where firing of the PIR continued from a battery powered breadboard, but in proximity to a halogen lamp --or was it a decrepit electrical system in a 60 year old tract house? However, my work shed where the problem first arose
was recently re-wired, including a new ground post. I didn't check outside, away from electricity, but plan to. I live in a desert climate with low humidity, for what that's worth. Static electricity floats around and occasionally, I have to ground my wrist just to type on my laptop.posting.php?mode=edit&f=24&p=112734#