As if ...were/had been

Your first sentence suggests that he is shocked at the same time as we hear him-- it is a condition at that time; the second sentence indicates that the shocking definitely precedes the sounding and perhaps no longer appertains but is a previous event.

thank you.

is it also something to do with "real" or "unreal"?

No, not here, although it is unsure whether he actually received a shock.

You are thinking of the unreal conditional:

'If he were married, he would not be smiling.'
'If he had been married, he would not have been smiling.'

Both of these express a hypothetical (unreal) condition. He is single; in the first he is smiling now; in the second he was smiling in

She looks as if she didn't understand what I mean.
She looks as if she hasn't understood what I mean.

could we use both of them, or is there a difference?

To me it seems a relatively straightforward reflection of the tenses in the dependent clause. The first is in the simple past: she looks definitely incapable of further understanding. The second is in the present perfect, and she may have a chance yet of understanding.

Practically speaking, of course, either might be said/written with the same thought in mind: she doesn't get it.

so, what should I do in order to imply that someone in fact, knows the truth, but hides it?

should I say "she behaves as if she doesn't know the truth." or

"she behaves as if she didn't know the truth."

I like 'she behaves as if she doesn't know the truth', which makes the statement more immediate for me. I can imagine someone using the second, however.

How about this?
She looked as if she hadn't understood what I meant. This is in past perfect tense.