This might be a difference in dialect. I mean ‘mustn’t’ as in ‘must not’.
Example sentence: “His shoes aren’t here. I guess he must not be home then.” —> “I guess he mustn’t be home then.”
This sentence is much more natural than “His shoes aren’t here. I guess he cannot be home then.”
This might be a difference in dialect. In New Zealand (and I assume other places, like maybe Australia, UK and Ireland) it is common to use ‘must not’ to mean:
a) ‘ Is forbidden to’ (the meaning you are familiar with),
and
b) ‘necessarily cannot’, often in a deductive way.
Example sentence: “His shoes aren’t here. I guess he must not be home then.”
This sentence is much more natural to me than “His shoes aren’t here. I guess he cannot be home then.”