Why is "de" used before "estar" in this sentence?
 

The sentence I have is:



"Hay gente que sabe que estamos aqu?, y ya deber?amos de estar en casa, de regreso de las monta?as."


I'm sure the sentence means "There are people that know that we are here, and by now we should be home, back from the mountains." But I feel like it would mean the same without "de" before "estar".

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.