I assume a bug in PS, as no one is editing the file when you aren't watching. Have you switched to APFS? Adobe seems to have problems every time Apple does something major under the hood.
Of course, if you assume the error message is correct, more interesting possibilities come up. You probably didn't know about the split personality business, and have been shrugging off the missing time. btw, have there been any unexplained murders in the area?