Update, I noticed an odd file in the dropbox folder starting with the letters scr and 10 numbers after it, and no file type. Had the same ArchiCAD icon so opened it from there and it was my latest file I thought I had saved. Great no lost work.
Perhaps I shut down to quick after saving and the computer didn't have time to change the previous save to .bpn and save the new version. Lesson learned.
Imac, Big Sur AC 20 NZ, AC 25 Solo UKI,