So here is the process.
1.) Open steam, on any account. Does not matter.
2.) Open steam in big picture mode
3.) Sign out.
You will be granted with all of your previous accounts before your eyes. Every last one of them, even if your user data was deleted for it.
To get rid of the account affiliation of your main (assuming you want better trust factor,) just simply click on the account(s) and click remove account.


Steps to clear Registry Editor data:
1.) Open Registry Editor
2.) Go to this directory: Computer\HKEY_CURRENT_USER\Software\Valve\Steam\Users\
3.) Delete all of the account data in which you don't want your main to have association with.
4.) Profit.

Example photo (some of these accounts I don't even remember....):