I took part in this thread because I had never heard or seen the use of that structure before.
I asked some teachers here and all of them reacted in the same way. I wonder what would happen if I decided to write mustn´t have been
instead of can´t have been
to show certainty about something in an international exam. I don´t think it would be accepted.
It might not be accepted if the exam is marked by a non-native speaker, if it is as unknown as you say.
I'd pass you, if that's any consolation.
I´ve found this use in another grammar book:
mustn´t have = necessity in the past
e.g.: You mustn´t have had any driving convictions or you won´t get the job.
Yes, that's a different meaning. Less common than the other one.