The feature everyone is referring to here is Threat Image Projection.
In Australia at least, operators are required to identify 'TIPs', maintaining a certain success rate. Dropping below this rate requires retraining, and if the problem persists, results in termination.
I first noticed the feature when I had (only) my car keys in one of the x-ray bins, but I could see that the screen was also showing a handgun. The screener said something like "well, that one was easy", and being the only person in line, I asked a few questions about it.
If they believe they are seeing a TIP image, they must indicate this to the software by pressing a button on their control pad. If they are correct, the 'fake' image disappears, and they are supposed to re-check the entire image for threats before proceeding.
If you're curious, you'll find more details here:
Rapiscan Threat Image Projection | Rapiscan Systems
Here is a screengrab that is available to the general public on the Rapiscan website: