Motion Detector has started to false alarm in the zone.

I have a Motion Detector that has started to false alarm in the zone two day's in a row. I've checked the battery and it's still good. And all of the others which have been working for the last three years are fine. I  try to trouble shoot it by changing it out with one in another zone and then had to deactivate it from the system so it's a mystery. Any ideas about what went wrong as it wasn't ever in direct sun light or near any air registers or pets? Thanks in advance.