There's a difference between '
deber estar' and '
deber de estar'. And we're talking about whether the preposition '
de' follows '
deber' or not.
Simplistic answer:
The first indicates obligation, like 'must be'.
The second means supposition, as in how you used 'should be'.
Not as simplistic:
The first can also be used to convey supposition, just as English 'must be' can mean both obligation and supposition, depending on how it's used in context.
But, '
deber de' always conveys supposition, the preposition following '
deber' ensuring there's no ambiguity in meaning.
Debe de estar en casa. =
Supongo que est? en casa.