So, thanks to the help of @wombatstampede i found the answer:
The solution was a dictionary, which can be accessed by a script of every individual chest, via a singleton. Each chest then just needs to check this global singleton dictionary. The distinction happens through the names or "IDs" from each chest, which is different every time a chest-scene is being instanced. This can be done just by calling the global method from a chest with the keyword "self":
Then, the chest "knows" the items, which belongs in itself. Adding or removing items is just the manipulation of that dictionary.