PIR sensors such as that model are not measuring a change in heat for the “motion detector” part. They are measuring a change in heat that moves across the sensor detection area.
In the winter, a common reason for false alerts for this type of sensor is a gust of warm air from the heating system blowing across the detection field. If you lay the sensor facedown, you can get the same change in heat, but it won’t be moving across the detection field, so it won’t false alert.
That said, over the last 12 months there have been many reports in the forums of false alerts from the current generation of the SmartThings branded motion sensor. It’s not clear why, it may have to do with the power management, which also seems to have some issues. Sometimes just changing to a different brand of battery seems to fix it, which is weird but possible.
Anyway, two years ago I would’ve just pointed people towards the FAQ, and felt pretty confident that they would be able to resolve the false alarm issue with one of the suggestions there.
Now, though, for this specific model there just seems to be a certain number that mysteriously generate false alerts. 