The saved email does not go to the Drafts folder unless you deliberately put it there. The auto-save runs automatically and the message is saved in a clipboard to be used in the event something unexpected happens. To see how it works, try an experiment:
Start composing a new test message and type in a few words then stop and keep your eye on the lower right corner of the screen. After about 8 or 9 seconds or so you will see the word "Saving" flash for a split second, then it will go to "Saved" and stay that way.
Now, close the tab as though you were doing it by accident. That will also sign you out of your email. Sign back in again, and underneath the folder pane you should see a tab that says "Mail: subject or "no subject" if you didn't have one yet. Click on that and the message you were working on should open back up. If you want to save the message to Drafts, click on the "X" in the upper right corner as if you are going to close the window, and you will be prompted to either discard it or save it to Drafts. You can do the same thing by clicking on the "Discard" button at the bottom of the window.
ETA: This works even if you have multiple Compose windows open when there is a power outage or whatever. A separate tab will be shown for each window you had open.