Documents are marked as changed immediately after opening in 25.4.1.
Since updating to 25.4.1, many documents are marked as having been edited immediately after opening them, instead of after actually changing anything.
I have not yet tracked down if there is anything documents doing this have in common. The more effects they have in them, the more likely they are to have this happen.
I've attached one of the simpler documents that's displaying this behavior.
OKAY so this seems to have ended up being a bug with Astute's WidthScribe plugin, they just released a beta update and this has stopped happening.
Sorry to have blamed Adobe, this one showed up after I updated AI, and was happening on files that weren't even using any of WidthScribe's effects! :)
The mysterious change seems to be the addition of four paths, and a group containing one path, to the topmost layer, immediately upon opening the file. These paths do not seem to exist in any meaningful form - I can't see anything when I select them, even if I zoom out past the entire pasteboard; if I turn on "zoom to selection" and try zooming in, the preview doesn't move from wherever I happen to have it centered.
This file contains nothing but these paths. If you delete them, save, and reload, they won't reappear.
Here's a file that I spent a little time deleting stuff out of until it stopped doing this. This is the last version that does it; if I unlock the single layer and save a new file, the new file will not be marked as changed when I load it back in until I actually change something. But this file? Instantly marked as edited the moment it's opened.
Here's another one that's doing this for me. It's super simple, just a few layers with some stuff sketched with the Pencil tool, and some translucent shapes with Gaussian Blurs on them to rough in some lighting.
Interestingly enough if I delete the layer with those blurred shapes and save this as a new file, it's still showing up as edited the moment I load it in.