InDesign thinking document is modified immediately upon opening (just started happening today).

So I think this is related to this other issue:

Solved: Indesign file flagged as modified right after open... - Adobe Community - 11317361

Seems to be triggered by placed PNGs. I haven't noticed this behavior until today.

Every time I open a file that has placed PNGs, InDesign flags the document as modified, with the asterisk. To give even more credence, it only happens when I navigate to a page that has a linked PNG file in it. If I open a file and there are no PNGs on the page, no asterisk. The minute I "page down" and hit a page that has a placed PNG, it "redraws/re-renders" the PNG (I have high resolution previews turned on, and when I get to those pages, the PNGs look a little low-res at first; you can see the dark blue progress bar at the bottom appear as it redraws the preview and then the image looks better once the hi-res preview is finished drawing) and then, BAM, asterisk appears. Nothing was modified other than InDesign redrawing/updating the preview.