Ninety-nine times out of a hundred the photos are in fact in the Catalog somewhere, even if not where you expected to see them. Any reorganisation on disk that LrC does not know about, even just renaming a folder, is not automatically reflected: the Catalog would still show them at the old location, knowing no better. The fix for that would not be a re-import - it would be informing the Catalog about folder(s) "change of address". Re-importing would in that case be highly counter-productive.
If you know that the image already exists on disk where you want it, and you also know that it is NOT imported anywhere at all within your Catalog, you can use the ADD option of import instead of COPY or MOVE - these options are shown at the top of screen when importing. That avoids producing any further duplicate of your source file (the camera Raw or the camera JPG).
But in those ninety-nine times out of a hundred, this would not be needed. Images don't just delete themselves from the Catalog.
Second, images do not just remove themselves from a Catalog. It may be that the particular view of the Catalog that you have, is not showing them. As an example, by default immediately after you import a batch of photos LrC displays that batch. But that does not persist: on another occasion different photos will be seen or none, when viewing this special collection. Instead, their folder or else any Collection(s) these photos may have been added to, must be viewed instead. Or you can view All Photographs and then they will (necessarily) be seen somewhere. One would filter down to help so it is necessary to use another way
By @richardplondon
That might just be the solution I was looking for. Thank you.
What I found was that the last time the catalogue file has been modified dates way back. So I think after the last import I somehow failed to save the catalogue accordingly. That might be why the files are on the disc but not in the catalogue.