Is there any way/place to store things reliably. There are lots of quest items that i dont want to carry around taking up space like a robot doll and the thing to scan the fence.
There is a place which you can rent and is 100% safe for storage. You will come across it in the course of the game.
Any other place is unsafe. However there are places to find and things to do to make your things more safe:
If you choose "Hiding" skill, it will let you see "concealment" bar in camp locations. Higher the bar, less likely will NPCs find your stash in those places.
If you don't have that skill, there is no way to tell this stat but generally administrative buildings and storage shacks tend to have concealment stat higher then other places. But there is no way you can be sure since it varies slightly from place to place. Crio chamber tend to have it also decent.
Presence of tarp shelter and fire lowers concealment stat of place/building significantly so newer leave one in the place you stored your things when you are away.
You can also make few things to not attract NPCs to your stash. Any thing placed on the ground (the default place in the hex) which cause "box" icon to appear at the hex attract looters and bandits. So make sure there are no such items there. Either destroy them or move them in to one of the camp sites. Rocks, glass, branches and some other things does not cause box icon to appear and are safe.
Clear also neighbouring hexes of any "box" icons.
If possible don't leave any containers (rucksacks, sleds, plastic bags, etc...) in the hex with stored items. If your stash is discovered, NPCs will use them to steal more things from you.
Don't leave containers anywhere as a rule. Only NPCs who collect and use them are those who can loot your things. Destroy containers you don't need or at last hide them if you want to keep few as a backup for later use.
Areas with lot of hostile wildlife are good because dogs and dogmen tend to clear area of humanoid NPCs and thus keep your stash safe.
Not all NPCs will steal your belongings. ANT warriors and DMC guards does not collect things, including your belongings. They also help to clear areas where they patrol free of bandits. However they don't attack looters (if I remember right).
If you follow those rules, your stash will be pretty safe.
Personally I've never noticed any campsite I set up being looted, although I make sure to never place stuff in campsites that provide no shelter or concealment. (I don't always use the hide trait to see the concealment bar on campsites, but you get a feel for where is safer after you do use it a few times)
Of course I'm sure somebody is going to say I've been lucky so far, and that sometimes camps get robbed.
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Yeah Ive made it as far as Detroit. What happens if i dont go back and renew the rental? All my stuff disappears?
Newer used that place so I don't know.
So you carry everything with you at all times?
Maybe someone should mod in a safety deposit box at the bank?
No I am storing things the way I described. I have half dozen stashes all over the map and only thing which I got ever stolen was sled which I hid temporarily in unsafe location (with low concealment value).
What happens if i dont go back and renew the rental? All my stuff disappears?
You will get a notification, and if you don't pick up your things in time they will get confiscated - which does mean you will lose them. That aside, hruza has given you all the info, that's basically how most of us handle loot stashes. With those precautions, having a loot stash here and there is relatively safe. Not 100%, but very very close, it will take some real bad luck for you to have your items stolen.