Car wash v. homeless

It is one of the mysteries of dog behaviour for me that Cimbi lies quietly in the car when we go to the car wash but he immediately stands up and barks when one of those alcoholist homeless beggars comes to the car. It is quite intriguing because at the car wash, before we drive into the machine, the attendants wash the screen with huge brooms. So nobody could say a word if Cimbi felt the need to bark. But he does not mind this, he does not even mind when my hubby talks to the attendant, pays, gets the change back. But when a homeless comes close to the car, he hates it and protests and jumps up and barks. Maybe he reads our reactions?