It is well known that one Cabinet Secretary is absent from the Capitol on State of the Union night to make certain at least one person in line of succession to the Presidency is not in the building. The order is generally the date that a Cabinet office was created so the Secretary of State is first (after VP, the Speaker of the House, and President Pro Tempore of the Senate) and the head of the newest Department, Secretary of Homeland Security is at the bottom of the list at 18th in line.
There has been talk of shuffling the order to more closely reflect the activities of the Departments like moving Homeland Security to just behind the Attorney General in 8th place ahead of Education, Agriculture, Interior, VA, etc., but old habits die hard and it hasn't gotten anywhere.
Since shortly after 9/11, the House and Senate have each sent a member of each party to an undisclosed location to assure continuation of those two chambers as well.
This last bit makes for some strange bedfellows. In 2006, for instance, Republican Eric Cantor (R-VA) and Democrat George Miller (D-CA) were selected to absent themselves from the Capitol Building.
I would pay good money to hear the conversation in the bunker between those two.
In the end the only two people who know the exact contents of the State of the Union address are the President and the aide who types the final edits into the Teleprompter.
The rest of Washington's most important people have to wait, just like you and me, to hear what he says.