The confrontation between Devoe and Dusan Gavric in the alleyway.

I have a question about that scene:

Dusan found himself blocked by a metal gate at the top of a flight of steps. Devoe, who is in the alley below, points his gun at him and tells him its over. Dr Kelly is just behind Devoe. Dusan's brother, who killed two cops who had previously confronted Dusan, was behind Dr Kelly and was about to shoot. Then Devoe turns around and kills Dusan's brother, saving Dr Kelly's life.

My question is: How did Devoe know that Dusan's brother was behind him and Dr Kelly?