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1. You will not have heard of me before but I used to live next door to your sister.
2. That is absurd; they really could not have taken your motorbike by mistake, surely?
3. You are a bit overweight; you should have been doing more regular exercise.
4. You were to have been promoted.

questions:
1. Why using this modal: will; why not You might have ... ?
2. Why is there a question mark at the end; why is it not a full stop?
3. Why using present perfect continuous? why not You should be doing or should do ... .
4. What tense is it?
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Comments  (Page 2) 
CalifJim,

I repeat my excerpt from Webster's Third New International Dictionary:
will verbal auxiliary
5 -- used to express probability or recognition and often equivalent to the simple verb [Emphasis added.]
<that will be the milkman at the back door> = that is the milkman ...
<this house with the green shutters will be theirs> = ...shutters is theirs...
<she would have been about twenty when she married> = she was about twenty ...
<discovered a plant growing and clinging close to the rocks. This will be the walking fern or walking leaf -- Anne Dorrance> = This [or that] is the walking fern...
<glass that hides the pendulum will often display a fine example of primitive painting -- Ellwood Kirby> = ...pendulum often displays a fine example...
To my ear, saying "you will not have heard of me" is just another way of saying "you have not heard of me" or "you don't know me." I don't sense the probability. It's just a statement of fact, perhaps in slightly elevated or polite language.