Learn Portuguese (Portugal): Dizem que o inspetor terá chegado atrasado à cena. - They say that the inspector must have arrived late to the scene.

In Portuguese, the Futuro Composto (Future Perfect) is frequently used to express a guess, probability, or rumor about the past. While terá chegado literally translates to "will have arrived", in this context it means "must have arrived" or "probably arrived".

Could I say deve ter chegado instead of terá chegado?

Absolutely. Using dever followed by the past infinitive (deve ter chegado) is another very natural way to express "must have arrived". The Futuro Composto just sounds slightly more formal or journalistic, which fits perfectly when reporting on an inspector at a scene.

Who does Dizem refer to here, and why is there no pronoun like eles?

When a verb is in the third-person plural without a specific subject pronoun like eles, it acts as an impersonal statement. It translates to "People say" or "They say" in a general sense. If you included the pronoun (Eles dizem), you would be referring to a specific, previously mentioned group of people.

Does the word atrasado change depending on who is late?