The Callas Blog has published an article about the necessity of «context aware” preflight. The new «Sifter» technology in Callas pdfToolbox 10 allows to analyze PDF objects in their context. See also my article A Quantum Leap in Preflight!.
Akash Choudhary describes some use cases in his post. The classic example is certainly small black text that is set to knockout:
If the black text is places over a colored background (left), overprinting is important to prevent white flashes in case of register inaccuracies in the printing machine. If the text has no background (right), it does not matter whether it is set to overprint or knockout.
With this technology it is now possible to find (and if necessary remove) completely or partially hidden objects. One can also check whether certain objects are placed too close to other objects (e.g. text near the TrimBox or the cut contour).
Checking individual objects against their environment is of course time-consuming and should be used with caution. But the loss of performance is particularly worthwhile in automated workflows, as the reduction of «false positives» can considerably increase the accuracy of the preflight results. This can significantly reduce the effort for manually checking preflight results!