So the problem has to be with the motion sensors...batteries good.
Are you sure about the batteries? If the system has worked since June and suddenly starts false alarms I would replace ALL batteries first (including the back-up in the console). Don't rely on the system to warn you of low battery.
If you have done that, check the system during the day to see if the change in season and sun angle could be putting fast changing patterns of light /dark on any of the sensors. They look for movement of heat / infrared, and should be fairly immune to light change but I have a MS on front porch (not a Security type) that changing sun patterns through the tree leaves will set off under the right conditions.
Finally, call X10 Customer Service to see if they have any wisdom (don't laugh at that. Someone there has to know something, sometime).