Thanks for your reply, but I don't think that is it. Mine is still set to "5 Minutes." I have just done something with my test account and edited it after less than 2 minutes and it still created an edit history.
The reason I'm asking, is I know there must be a (very short) period whereby it is possible, because I had a new member make a new thread which appeared straight away. This person then edited their post and added a word which triggered the Spam filter, sending the thread into the Moderation queue. A Moderator asked me how a [non-moderating] Member viewed this thread; upon checking the Spam trigger log I found that it was triggered from the edit screen, but with no edit History shown.