as far as i can find, this does not exist.
basically, it would be designed to stop the motion detector from going off from things you are not trying to capture - trees, shadows moving etc.
the way i envision it, you set your grid resolution, then set it to 'auto mask' for a specified time, and during automask, any cell that detects movement is 'masked'. At the moment it is only possible to mask whole columns or rows, and this is hard to mask if there's something in the middle of your pic setting it off. plus sometimes you can't tell what's setting it off, which is very frustrating