Look at the shadow on the front door when the PI opens it. The contours clearly shows a person with a full set of hair which excludes Bertram, Stiles and McAllister. That leaves us with Haffner. Also, remember that Haffner didn't have time to say goodbye to the CBI team. Wonder why?

PS. The reason I exclude Smith is because of his weight. According to Rosalind it can't be him.