I've built a 4WD robot that autonomously patrols my garden at night to ward off rabbits. It patrols for a couple of minutes then sleeps for an hour then repeats hourly. I then added a PIR sensor such that on detecting motion while asleep it would start to patrol (don't want the rabbits to learn its sleep pattern and start to take the mickey!!) . I've tested the sensor/code indoors and it works perfectly. However when I deploy it at night outdoors the PIR is being triggered all of the time despite there being no heat source nearby. There is a street lamp at the corner of the garden, could this be the heat source that is triggering the sensor ?. It appears to function correctly during daylight outdoors therefore it has to be to do with the night conditions. I could possibly try some form of shield around the sensor but this might limit its effectiveness too much. Failing that I'll just buy a shotgun 
The street lamp and the wind moving foliage will certainly do it. I have a trail camera that sometimes does the same thing when the wind blows on tree branches that have a tiny amount of light from the neighbor's yard light. Car lights from a nearby road also trigger the camera at night if it's not carefully pointed away from the road. You are dealing with a sensor that sees most light, including IR.
Paul
Thanks Paul. It's difficult as the sensor isn't static therefore the lighting conditions are variable. I'll try shielding it though to see if I can desensitise it sufficiently without rendering it useless,
Robin