To your knowledge, are all the images listed in the database?
The list of which magazines were used are complete, but not every magazine is available, for whatever reason. No doubt they are chock full of aliens, astronauts sat around smoking on set etc etc.
The missing ones tend to relate to orbital science or specific experiments, such as the Hycon images Awe130 was banging on about from Apollo 14, or the 35mm photos taken by Apollo 15 of stellar targets.
Isolated examples of these images do appear in scientific and technical reports, so they were processed, but they haven't been scanned by anyone for the modern age.
Your mystery image from Apollo 16 is listed in the Apollo Image Atlas as 'Gray Scale', so it's likely that that one, and others like it, were taken for calibration purposes to help with analysing other images on the magazine.
I've heard Marcus Allen claim that 32000 images were taken on the moon (and he specifically uses the word 'on'), but this is way over the actual total of all images taken (the Image Atlas cites 25000 lunar images including Metric and Panoramic camera images). Most of the images are taken in lunar orbit, and a substantial number in LEO and cislunar space. The majority of surface images in Apollo 15-17 were taken during LRV traverses.
Another hefty chunk accounts for photographs of samples and or experiments 'in situ' and 'locator shots' to give a context for samples and experiments. Very few Apollo lunar surface images are what would be described as 'tourist' or 'opportunist' shots that average HB expects, mostly because the film budgets were very carefully worked out in advance.
