The Reengaged contacts graph is the first place you are likely to see the impact of using send time optimization. It shows the total number of contacts that were dormant 90 days prior to and after using send time optimization and are still dormant. You can download the list of re-engaged contacts by clicking the Download link in the chart footer.
It's not uncommon to see contacts who have been dormant for years "wake up" after turning on Seventh Sense. If you've been sending at 10am on Tuesday for years but a contact works night shift or lives overseas, they may not have seen one of your emails at the top of their inbox since they subscribed. but if they opened just one email in the past, Seventh Sense will use that data to try to re-engage them.