Graham, you're being phenomenal again. What a treasure!
Here's some info that looks like it'd be helpful, from a preliminary read-through:
There are official PDFs available for direct download, that don't come up when you search for them in the Building Instructions dialog. They are not included in my posted count of PDFs available, because they're not readily identifiable and so the total number of PDFs would be confusing. However, by elimination, here are the sets with stealth PDFs:
PlaymoDB List - PDFs available but not searchable.
I keep track of sets that have no instructions, by assigning them the fake part i0000 (Instructions Do Not Exist). It appears in 679 sets in the DB. Now, some of these are assumptions (e.g. specials, eggs) but a fair number are based on someone telling me or posting a photo of the full contents of a freshly opened set. Here's the link to the fake part, and the longest list of "sets containing this part" ever.
http://playmodb.org/cgi-bin/showpart.pl?partnum=i0000Some error-checking...
The missing PDFs: 3038, 3050 and 3057 have never had any individual instructions of their own, in my knowledge.
3038 - Formula 1 Car is probably equivalent to
3603 - Formula 1 Racing Car (Yellow), which does have an official PDF.
3050 - Pirate Ship is probably equivalent to
3550 - Brigantine, which has instructions but not an official PDF.
3057 - U.S. Artillery and
3812 - U.S. Artillery share an instruction sheet, labeled with both numbers although 3057 is denoted as "USA/CDN". No official PDF.
9605, 9607, 9608, 9728 have always been part of the DB.
4930 - Boy with Tractor is a recent addition - you have its theme in PlaymoDB but haven't removed the "not in DB" label.
Now, to add those 158 sets not in the DB and totally mess up Graham's system

Thanks so much for finding them!