A security camera timestamped 17:42 shows Sami locking the empty office and leaving with his coat. The weather station records that the storm began at 18:10. He ___ before the storm began.

Explication

Must have left is the confident past deduction supported by a timestamped departure that precedes the recorded start of the storm. Must had left uses the wrong auxiliary, while mustn't have left is not the standard form for a negative deduction.