Must have (done) – (1)  – to express the present supposition /guessing of what would have happened in the past.

• We must have taken the wrong road. The sign board was not clear.
• She predicted that such a creature must have existed.
• It must have been dreadfully difficult.  How did you manage the situation?
• It must have been agonising for the people around you.
• We rang the calling bell but he didn’t come out.  He must have been asleep.
• You looked weak and pale.  You must have been very tired of the heavy work.( i.e I am sure you were very tired)

Must have (done) (2) –  to show that it is necessary for the first thing to have happened before the second thing can happen.

• In order to qualify for the full pension benefits in the Bank, you must have completed 33 years of confirmed service.
• If you are to be included in the list of eligible candidates for the benefits, the application must have been submitted before 31st of December.