file won't stay saved

I have an InDesign file that I know I saved before closing, but when I open it the next time, it shows that it isn't in a saved state (black dot in the red dot of the window) and if I close the file without doing anything, I get the message asking if I want to save it before closing. I've tried Save As, which sometimes creates a smaller file than if I just Save as usual, but that didn't help this issue. I also tried restarting the iMac; no luck. Suggestions?

ID CC 2018

Mac OSX 10.13.6

iMac (Retina 5K, 27-inch, Late 2015)

ps: OK, I just tried Save As and changed the filename by adding 1 character, and now it reopens in a saved state, AND the original file also opens in a saved state. Any idea what was happening before?