The Silversteins' and Silverstein Associates' "Luck" on 9/11
I didn't know this about the Silversteins missing the attacks, but I am hardly surprised:
Silverstein Properties took over the lease of the World Trade Center seven weeks before 9/11, the only time the complex had gone under private control , and after 9/11 Larry Silverstein sought damages of over $7 billion from his insurers for the destruction of the Twin Towers.  On September 11, the firm had temporary offices on the 88th floor of the North Tower. (American Airlines Flight 11 impacted that tower between its 93rd and 99th floors.) Of its 160 employees, 54 were working in those offices at the time of the attacks and four of them died. 
HIGH-LEVEL INDIVIDUALS AVOIDED DANGER ON 9/11
The fortunate circumstances that kept the high-level individuals out of danger the morning of 9/11 were quite extraordinary. Larry Silverstein survived the attacks supposedly because his wife forced him to go to a doctor's appointment instead of a meeting at the WTC; Silverstein's son and daughter survived because, independently of each other, they were running late; his top aide survived because he cut short a meeting he was in at the top of the North Tower; his publicist agreed to join a meeting at the WTC at 9:00 a.m. instead of 8:00 a.m.; and others associated with Silverstein Properties may have avoided danger due to the cancellation of a meeting on the 88th floor of the North Tower.