Gen 1 Motion Detector triggered by pets but only at night

I have just installed a Gen 1 system and have 2 motion detectors installed. Both are set to medium detection for small pets, and located at correct height and in hallways. During the day, our small cat can walk past the detectors without motion been detected and reported in the history. However at night, they frequently trigger a motion detection event. I have tried a reset of the detectors, changing to low detection (for larger pets) and a change in detector location. Any other suggestions?