Please see my comment and description of the bug here:
https://community.smartsheet.com/discussion/summary-report-multiple-sheets-modified-date?page=1#comment-19340
When auto-save after 3 minutes is on, a sheet with the TODAY() formula in use will auto-save on initial open after a few seconds.
While this may be a "feature", it needs to be divorced from the 3 minutes of inactivity feature.
Craig