A few image files disappeared: can't be re-imported from backup

Question

There are at least a 4 dozen pictures imported into LR 5.x last September that now cannot be searched and which are not in the dated folder where they should be - even if searched in Windows Explorer. It's part of an event, and it appears to be all those on one card from one body. I remember seeing these images in the Library module at the time. No Previews with question marks.
If I go to the duplicated import created that day I find the "missing" images in the folder created at that time. Yea Adobe for creating the easy back-up for importing. In this backup folder the images can be searched and opened in either Windows Explorer or a viewer like FastStone. .  If I put the files into the date-organized database folder (to "Synchronize" in the main directory) LR says it cannot import them as they are already part of the database. I tried copying these files in Explorer to a different hard drive and tried importing again: no help.
How can I convince LR that these need to be re-imported? More importantly, can anyone offer a scenario by which these image files disappeared from the disk and didn't leave a preview with a question mark? I didn't actually do any work (Dev module) that I remember on these. I certainly can go through with Bridge and batch rename this little set and present them again  as an import request. But what causes something to drop out of the DB and disappear as a file? Obviously some kind of operator error is the most likely, I understand.
Deleting all Previews? Seems like that would make the machine work for hours to redo Previews for 90,000 pix.

Answer 1

Thanks for the suggestions (and warnings). I am pretty experienced with Lightroom and know the structure behind it. What was weird here was that when I try to bring one of the images back in through Synch or Import, says "It's already part of the database" BUT when searching for the image by file name LR finds others that share the same file number (not strange) but NOT the image I want (strange). This search was performed at the root of the file/folder structure, so should be looking into every folder in the database. An Explorer search shows that the image is no longer in the database folders. It is only found in the "Backup" folders created by LR when ingesting the card at the original import.
OK, so use the OS to move some of the files (12) back "inside" the folder, click Synch folder and it tells me there are 12 new photos. So far so good. BUT -- although they are now "there" in the folder and re-imported they still do not show up in the Library thumbnail view and can't be searched in Lightroom (by file name). The 12 didn't show up on a import screen with one lone PS image in that batch that was newly created this afternoon and DID show as a new import. I expected to see the 12 on that screen.
To answer one question offered today: No, the images do not show in the Develop module.
So something tells Lightroom these are not new and that they are aready there. However, not searchable.
Next step changing the names of the offending file, like appending a letter. See if that makes them look "new".
I think I can fix this as long as I have to original CR2 files unprocessed, which thankfully, I do. Anyone have a theory about what causes this? Never had it happen.