The pernames.ntf is always found in the data directory, that's what was puzzling at first.
No, these are standard IBM Notes installs, using the IBM install package.
This issue, as I tried to describe it, is that upon restart of the client, the Notes.ini line: 'Directory=c:\Program Files (x86)\IBM\Notes\Data' had been changed to 'Directory=c:\Program Files (x86)\IBM\Notes' - the pernames.ntf file is NOT found there, nor does Notes work.
Changing the Notes.ini parm back fixes the issue - the question is - why does the notes.ini line get changed, and by whom?
When it happened the first time, it was odd, but we dismissed it. The 4th time, to a 4th user, seems like some kind of pattern.