As noted in the court decision, Hilo used the CTX 5500, which I have had years of experience working on while at SAT. Have you ever seen stacks, even small stacks, of photographs displayed on the CTX 5500? Not to be rude, but from your comment, I doubt it. Dark masses is exactly how such pictures - thicker and heavier than paper - would appear on the CTX 5500. In fact, get a large enough stack of such photos and it throws off the entire scan, meaning a hand search of the bag, as the density of the pictures will cause the scan to fail.

I will add that what should be common sense is when you have an "alarm" on a CTX you look at the image of where the items are BEFORE you begin the search. It will tell you if the item is near the top of the bag, at the bottom, or by the wheels, or the pull handle, etc.

So we can know that the screener knew the general location of the items she needed to check. She knew the "black mass" was near the opening. I would suggest the black mass was near the opening, and when she saw the pictures, knowing how they look on the CTX screen, it actually makes sense to someone that she could accurately guess those were the items that caused the alarm. Similar situations have happened to me. They have happened to anyone who has worked on CTX, or L3s, given enough time.

But I do understand that this would not make sense to someone who has no experience with this topic.