In Spanish, we use El Futuro Perfecto to make a supposition about something that happened in the past.
Se habrán acostado al llegar al hotel porque el viaje ha sido largo.They must have gone to bed upon arriving at the hotel because the trip was very long.
Lo habré conocido antes porque su cara me suena.I must have met him before because his face is familiar.
In English this supposition is generally expressed with "must have".
In Spanish we don't need to add anything to the sentence to express that supposition/probability. The use of El Futuro Perfecto includes that nuance.
The use of El Futuro Perfecto with this nuance will very often be triggered by a question or previous comment using El Pretérito Perfecto or El Presente for example:
"Marisa está muy guapa hoy, ¿verdad?"
(Marisa looks very pretty today, doesn't she?)
Habrá ido a la peluquería esta mañana.She must have gone to the hairdresser's this morning.
"¿Dónde ha ido tu hermano?"
(Where did your brother go?)
Habrá ido a casa de su amigo Miguel. Siempre lo visita los sábados.He must have gone to his friend Miguel's. He always visits him on Saturdays.
See also Conjugate regular verbs in El Futuro Perfecto (future perfect) and Conjugate irregular verbs in El Futuro Perfecto
For a different way to make suppositions about the past see also How to say "must have [done something] with the verb deber
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