To explain this question, we need to know the theory of how motion detection works.
The DVR system can detect small optical changes. It can be triggered by little image change from the camera. The image is transmitted by a light signal.
Therefore, even if there is no object in front of cameras such as humans, animals, or cars, the system may still be triggered by small light changes such as floating clouds, shaking leaves.
Most customers hope the system can only be triggered by a big object like people and ignore the small movement. But according to the theory of how motion detection works, there are false alarms. It is hard to say the system is faulty due to not only the Sannce brand system faces such issues, but also other brands who use a light sensor to work for motion detection function.
To reduce false alarm, you can change the sensibility to lower and set schedule alarm.
Or you can select a smaller area to detect movement.