So now I think it probable they were ghost files with size zero, which are probably gone now. However when I was emptying the trash I saw that they were in a group which I know to be old and unimportant so don’t believe I have lost anything.
OK I now have found these Ghost files in a backup I made of all my data before importing to DT in 2019.
There is indeed something odd about them because they were zero size at that time, as in this screenshot:
These are the names I saw in the Ghosts smart groups on iPhone and iPad.
Bottom line: they have been flagged as Ghosts because they were zero size, but the zero size was not caused by DTTG 3.