That is the problem I have been contemplating. All I can think of at the moment is going down a level, finding the same column, and assessing the situation from there. Continue downward another level if no exit from the column is found. Otherwise look for anything (or anyone) suspicious. Ask employees if we have too. Check for building exits, etc.
We really don't have the people to cover all the exits, and organizing people to do so, I suspect, will be difficult. Directly following the assassin is the best bet. Dawson is thinking that if they move fast enough, Al's plan to pincher the assassin can work. Dawson will take point down the stairs, drawing attention, and perhaps flushing out the assassin.
So in summary, directly chasing/tracking the assassin is the plan.