To explain this question, we need to know the theory of how motion detection works.
The DVR system can detect small optical change. It can be triggered by little image change from the camera. Image is transmitted by a light signal.
Therefore, even if there is no object in front of cameras such as human, animal or car, the system may still be triggered by small light change such as floating cloud, shaking leaf.
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 how motion detection works, there are false alarms. It is hard to say the system is faulty due to not only Annke brand system faces such issue, 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.