"Then, she must have been at the house."
Translation:Entonces, ella debe de haber estado en casa.
"Deber de" is used to express probability. In this sentence, another translation would be something like "it is very likely that she was at home".
"Deber" (without "de") is used to show obligation. She had to have been at home (to receive a package, to let the repairman in, to make sure no rowdy kids hang out in the backyard, etc).
Take a look at this article: https://spanish.yabla.com/lesson-Deber--Deber-De-%2B-Infinitive-197
Cut and pasted: Is it "en casa" or "en la casa"? | SpanishDict Answers www.spanishdict.com › Q&A › Vocabulary & Grammar According to one grammar book, it says it could be either; but "en la casa" is more common in Latin America. In another book, it says that it is always "en casa," unless "casa" is modified by a phrase to make it stand out, as in "en la casa blanca."...