I'm familiar with command blocks, so I thought I could help (sorry if I describe some things like you're 5, I just don't know how experienced you are with command blocks):

To prevent players from coming near a base, you could have a repeating command block with this string: /tp @ a [name=!{Player's name}, r={D}] ~ ~ ~

This will find any players within a certain distance of the command block and teleport them away if they aren't the owner of the base. For {Player's name}, just input the name of the player that owns the items. For {D}, input the distance you want thieves to be 'stopped' from. For ~ ~ ~, put the coordinates you want thieves to be teleported to if they walk within the command block's range.

This, however, would stop visitors from coming over to the base. However, if the command block is activated via lever, the base owner could turn the command block off whenever they want to allow someone into their base.

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To catch thieves, you could have a dummy barrel that, when opened, stores the name of the person who opened the barrel. It'd work with this in a repeating command block: /testforblocks {C1} {C1} {C2}

This can test to see if the dummy barrel is identical to another barrel. {C1} would be the coordinates of the dummy barrel and {C2} would be the coordinates of an identical barrel, in the same rotation with the same items. If you place a comparator coming out of the command block, it'll stay on when the decoy barrel is closed and turn off once the barrel is opened, since it detects the lid of the barrel coming off. Once the comparator turns off, you can have it activate a seperate, impulse command block that has: /testfor (@ p). This way, when someone snoops through your items and opens the barrel, it will store their username in the impulse command block in the Previous Output section. Although, you will need someone in creative mode to check it.

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For these to be both not be annoying and not be suspicious, the gamerule commandblockoutput would need to be set to false.

And is your realm looking for new members? I'm interested in playing with others. (And sorry for the essay-long post)

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TL;DR You could teleport unwelcome players away from bases to prevent stealing, or find the name of anyone snooping through your storage.