objects that are not there

Just do a 'Normal' mission over the area with 80% overlap in both directions. This will ensure that you are getting enough overlap for a reliable model. Upload the images to Maps Made Easy for processing. Then you can make the volume measurements as noted here:

https://support.dronesmadeeasy.com/hc/en-us/community/posts/205200883-Repeatable-Volumetric-Measurements