This isn't Open Carry as such because most security guards have no option to carry concealed.
I'd bet that he was at THAT location every day, giving the thief ample opportunity to plan his attack.
I know quite a few security companies that require their guards to carry with an empty chamber.
Just in general why not try to OC spray the thief?
Why wasn't the guard at the TOP of the stairs? In a place where there's only one avenue of approach?