I've run through the "doomsday scenario thing" in my head a few times myself and have questions...

You buy a shelter. Be it a standard cabin out in the woods with solar panels / wind mill, or underground steel and concrete re-enforced armored bunker, do you have so much as even a two track road leading to it? Yes? Then people
will find it!
You will be killed by looters.I know, I know, everybody's thinking. "NOT ME! I'VE GOT GUNS AND AMMO FALLING OUT MY REAR!"
Won't matter. If you ever leave that shelter
you will be killed.Gotta dump your waste? Gotta hunt food, fish, or tend crops? Gotta check on why your power source isnt working (solar panels, windmill, whatever. Yes, I shot it)? How long can you hold out without opening the hatch? Open the hatch = you will be killed.
Everybody take a deep breath and cool off. I've proposed this scenario to MANY, MANY people, and have only gotten a legit solution once, after hours of discussion, but lets run through some idea's we can all learn from. People get hot QUICK when you shoot down their "fool proof plans", so everybody please dont get bent out of shape, lets just learn.
NOW. Lets suppose I can aford a very fine high powered rifle. But NOT a very fine shelter or supplies. Quite a differance in cost here! I like any person want to survive, so what do I do when things are very bad looking for me?
While specific location makes NO differance, lets say I drive up north (I lived in MI for 14 years and just moved 2 weeks ago, so I know the U.P., & Northern lower peninsula well!), I look for remote, long two track roads and drive down them for a quick investigation much like MANY, MANY others looking for food / help will. I find at the end of this road a cabin, shelter, man hole and vents for an underground shelter, whatever the situation may be, and drive away. Park my car blocking the road (dont really care what happens to it at this point), put on my ghilli suit, camo up, and stalk my way back to a good shooting position as far away from my target (your door, manhole, access point) as I can position myself with a clear shot with my sniper rifle. I have enough supplies to last a week or two, and simply wait you out... Up pops a head, and boom. I wait two more days before aproaching my new found treasure. If others should pop up. Same result. Your family WILL burst out of the shelter into open fire having seen you shot down. Its called panic. But again, I wait two days to be sure.
I stock up on your goodies and leave. Walk a FEW miles, and find my next target.
I know its ugly. But you REALLY believe MANY people wont be doing this!?!?!?! What's to stop me from years of this living this way??? There ARE answers.