The objection is that they would have been picked up while moving under also-moving cover.
Subtractive imaging shows both objects that are gone, and objects that are new. You just use the absolute value of the result.
Essentially, for a thresholded image, it's:
abs(1-0) = 1 // object has moved away between images
abs(0-1) = 1 // object has arrived between images
abs(1-1) = 0 // nothing has changed
abs(0-0) = 0 // nothing has changed
Image polarity, greyscale, color, bilevel and so on make it a little more complex, but only a little.
In English, if you absolute subtract two aligned images of the same region, everything that is the same goes to zero. Anything else shows up as a brighter spot. The bottom line is, you can't hide something moving unless a satellite imaging system can't see it at all. Not the case with large rocks, I'm afraid.