Motion detectors going off for no reason

My Motion detectors are going off for some unknown reason. They are located in the garage, and they are being triggered randomly when I am certain there is no human in there causing the motion. They are triggered repeatedly several times within minutes, and both the batteries are fully charged. Other potential factors I was wondering about include the temperature beingVery cold, dust, or potentially a rat or mouse, although I do not see any droppings from the such critters. Does anyone have any experience with the above factors triggering your motion sensors or have any other thoughts on what could be triggering the sensors? Thanks so much