In an Oxford dictionary, ‘must have –ed participle’ is used for supposing a past event.
He must have known (= surely he knew) what she wanted.
I'm sorry,
she's not here. She must have left already (= that must be the
explanation)
(Advanced Learner’s)
However, a case below seems to have some different meaning. Although, yet, I do not have found the explanation, from a grammar textbook for Korean language, I guess the case seems to express an awareness of realization, perfection, or the continuity of the perfection at present with a past form. Can my guessing be right, or am I to learn about some other explanation?
“Jane,” he recommenced, as we entered the laurel walk, and slowly
strayed down in the direction of the sunk fence and the
horse-chestnut, “Thornfield is a pleasant place in summer, is it
not?”
“Yes, sir.”
“You must have become in some degree
attached to the house,—you, who have an eye for natural beauties, and
a good deal of the organ of Adhesiveness?”
(Jane Eyre)
Best Answer
Your guess is correct. The present perfect can be used:
1.) to refer to a completed action in the past.
2.) to refer to a state that began sometime in the past and continues into the present.
The same applies when the present perfect is used with "must" to indicate a deduction:
She must have left = I deduce that she has left. [The leaving is completed.]
She must have become attached. = I deduce that she has become attached [and still is].