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.