Sentence is fine. Say "she" left the party about a half hour ago and her apartment is only two blocks away. " Someone could argue that if you don't know she is home, then you can't use "must".
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Suddenly two white lights flashed while they haven't stood vertical completely. "Bang", the two swords bumped together. Both of them stood back a step. The onlookers around all voiced "Blimey!".