The reason that footage may be disappearing is due to the nature of loop recording, which overwrites the oldest files with newer ones. This is done to ensure that the dash cameras can constantly record endlessly, unless you make an effort to protect and lock the footage, which will prevent it from being overwritten. To learn how to lock a recording file and protect it, follow the guide in the link below:

NanoCam Plus Dash Cams - How do I protect loop recordings? :