Must have been can have two meanings— it depends on the text.
Must have p.p. doesn't always mean that, for instance, something did not occur— it might occur.
For instance:
Clerks must have been in the garden.
Clerks who have to be in the garden, but they were not.
Clerks who have to be in the garden were in the garden.
Am I right?
Must refers to a necessity.
In the present or future tense, it can sometimes indicate a matter of fact ("what goes up, must come down") and sometimes of obligation ("you must pay your taxes"), or merely exaggerating what is not a necessity by describing it as such ("you simply must see the production of Coriolanus at the Donmar Warehouse if you're in London, it's superb")*.
With the perfect tenses, must indicates a matter of fact, that has been deduced.
He must have been here recently, the kettle's still warm.
Now, "matter of fact" does not necessarily mean it's true; but if it isn't true, it's because the deduction is incorrect: We're confident of our deduction.
It can also be used to state a deduction of what must follow in a particular case:
If he had been to the club, then he must have seen her.
This puts forward that in a certain case ("he had been to the club"), then a certain fact would follow ("he saw her"), though in itself it doesn't state that he had been to the club, so it allow for both parts to be false; again what is expressed is a deduction about how one thing would follow from another.
*And it was, but it ended last Saturday.
I am confused by what you are asking. But what I think you are referring to, are the two meanings of: 'Clerks must be in the garden'.
It could mean that because they are nowhere else to be seen - 'they must be in the garden'.
The other possibility is that it means 'clerks have to be in the garden', i.e. they are somehow acting contrary to their employer's wishes if they are not in the garden. e.g. 'When the fire alarm sounds, clerks must be in the garden within 2 minutes.'
Now if you want to use the form 'must have been', it works with the first example: 'If they were not at their desks, clerks must have been in the garden'.
But it doesn't work with the second example. In that case you would have to say 'As the fire alarm had gone off, the clerks should have been in the garden'.
Everything depends on context. Be that must have been or any other phrase in the English language. That said, must have been is used when you see some clear evidence and are sure that the thing you are going to talk about has actually happened, but you are still not 100% sure. Here are some examples:
The lock is broken. Someone must have broken into the house.
There were papers filed in our rosebushes this morning. Clerks must have been in the garden again. – Peter Shor
In these examples the evidence shows what you suspect, but there's a possibility that could prove your conjecture wrong.
