I believe that, to keep confusion down, there is a few minutes (I think 2-5) of free edits before the boards starts counting them as edits.
However, I believe this only counts for your own edits within the next few minutes. If I mod edits your post, regardless of the time, it should pop up on the bottom of the post.
Also, a mod is required to leave a note in a post they edit so if a mod edits a post, you'll know. The only times this doesn't happen is when the post is outright deleted, in which case you will receive a PM if it was for anything important.
Hope that helps.