Not a teacher, just a language lover.
If you say "can be" or "could be" then still people can see what you mean but I think "could have been" is the most natural way to say it. That's because the speaker is talking to something not real. He "could have been" at home but actually he is now NOT at home and the fact that he's at home is not real, just something like a wish.

Student or Learner
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