Solve the mystery #19

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Dr. Hall was the only customer in the little drugstore when the shooting started. He had just taken his first sip of black coffee when three men dashed from the bank across the street, guns blazing. As the holdup men jumped into a waiting car, a nun and a chauffeur sought refuge in the drugstore. "You're both upset," said Hall. "Let me buy you a cup of coffee." They thanked him. The nun ordered black coffee, the chauffeur a glass of root beer. The three fell to talking about the flying bullets and had barely touched their drinks when sirens sounded. The robbers had been captured and were being returned to the bank for identification. Hall moved to a front window to watch. As he returned to the counter, the nun and chauffeur thanked him again and departed. The counterman had cleared the glass and cups. "Sorry, mister," he said to Hall. "I didn't know your weren't done." The counterman looked at the two coffee cups he had just removed from the counter, and passing Hall the one without lipstick, said, "What do you think a chauffeur was doing around here? There isn't a limousine on the street." Hall thought a moment. "Good grief!" he cried. "We had the gang's lookout right here!" And he dashed out to make the capture. What aroused Hall's suspicion?