I get the same thing, when saving an .ai file to Dropbox. doesn't always happen, but all the files are created new in CC2108 and only used by myself and also only on one desktop--so it has not been accessed by me or anyone else from another computer. I am running Apple OS 10.13.1 and Adobe CC2018

Only started after the CC2018 and 10.13 update, so something between those I would think

Same problem here when saving an already created AI file: “this file has been modified outside illustrator.” It started happening when I updated Mac to Sierra and High Sierra. Any solutions?

...should have also said, I am saving .ai files. It only happens when I'm saving a file I've been working on.

Having exactly the same problem. All my live work is in the DropBox folder on my MacBook which is constantly synced with DropBox servers—assume it's something to do with that (perhaps DropBox flagging the file as updated when it's synced, which would happen in between each save).

Only started after updating to CC2018.

Getting the same issue with saving of existing .ai files.
It will save fine without a message and I will only have to move one thing in the document, try to save it again and i will get the popup: "this file has been modified outside illustrator" the next minute. I save to a dropbox folder and the problem started after upgrading to high sierra (OSX 10.13)

I have the same problem with 2017 and 2018 AI. It happens to my old files and new files but not all the files.

I'm getting the same issue after updating to OSX 10.13. I updated Illustrator to 2018 version but it is still doing it. I see it mostly when I'm saving my files in .AI format.

This morning I updated to Adobe 2018 and the problem still persists. I can move things in my drawing and save but if I add to the drawing it throughs up the message about modified outside of illustrator and so to be safe I go ahead and do a save as. Very strange.

Also it has started the problem with the save as with my 2016 MacBook Pro. As always I save as ai files. I have tried resetting illustrator which fixes some of the pallet problems but not the save problem. I can work around but its just vexing. Thanks

I get this message when saving as ai file to my desktop that is shared when another computer using iCloud. The message does not appear if I do a save as only when I just hit save. This started when I up graded to Mac OS 10.13 on a 2015 iMac. My 2016 MacBook Pro doesn't give me the message, possibly because when I use it it is only connected to iCloud and no other computer at the time. Sorry for lack of clarity.

I get the popup when saving to an AI file. I get it when saving to a Dropbox folder but only I have access to that folder.

I have this problem as well. I can't reliably reproduce or trigger it but it happens. Nothing to my knowledge has changed the file.