Some newer (in the last 10 years) smoke detectors use an infrared sensor to detect fire as well as smoke, and it may be going haywire off a reflection of the sun, or intense heating of a spot within its detection area.
If you can, borrow a FLIR or infrared camera and check the area when the detector goes off.
If you post your model of smoke detector, it would be easier to tell if it has this feature.
In the US, you own the airspace up to the highest point of your property, including structures (and maybe trees). You also technically own your underground property to the center of the earth, but you may not have mineral rights to it.