A feature of Buddhism is Noble Silence -- i.e. not getting drawn into speculation (or "unwise attention").
I don't want to complain about the question, but the first paragraph is fiction (referencing the Matrix) and the second is like a "thought experiment" but I don't know what you're saying with it or if it's a metaphor for something -- it appears to invite speculation (maybe someone took it? maybe you misremembered?) -- or it's perhaps a paradox.
There are Kōans, used in some traditions of Buddhism, I don't know that's related to this question though.
One or two suttas mention the four great references -- and according to that I'd say that your question is not the kind of thing that the Buddha talked about -- to that extent "Buddhism" doesn't say anything about that topic.
At the risk of sounding too new-Age I might add that buddhism points out that put-together things are impermanent -- and so are thoughts, that there are different thought-moments. Given that your second paragraph is no more than a thought-experiment, there's nothing very remarkable about a thought (like "a key") existing in one moment and not existing in another.