I just recently managed to find all of the Keller tapes and Bobble-heads, I have also killed all the super mutant behemoths, all locations have been discovered and to the best of my knowledge searched, is there anything left to discover? Secrets, etc........?

Have you got any of the DLC's? if not thats where i'd start... great fun that lasts hours

secrets are all around us, lurking, preying for weakness.

Well, there are some special PA that you can find, pretty hard to find them 2. i always give them to my followers, cause they look special, if they got different type of PA on.
BTW be better if u told us what u have found, saying u probably searched everything leaves this big gap in your question.

I'm on my twentieth character, and just stumbled across a little shack out in the Wastes... never been here before. And never seen such hostile females either. So I had to kill them. Sorry.

If you're a PC player (and I'm assuming you're not), you have lot's left. And if you're a console player why don't you start a new playthrough? You're bound to find something new that you've never found before as there is a lot of http://fallout.wikia.com/wiki/Fallout_3_random_encounters. If that still doesn't please you then how about choosing a different path this time?