Contact sensor battery life very poor

I have one contact sensor that is using it's battery at an alarming rate (already failed within 4 months), compared to the rest of the system. It is on a window that is never opened, and located between several other windows that are still near 100% battery. Had anyone seen this type of behaviour? Since installation, I have already replaced the batteries once. I can come to no other conclusion then the sensor is somehow defective.