Thank.

Perhaps he would say "Yeah" in English. "Jo" instead of "Ja" here seems to indicate that he might have noticed something. We would say "Yeah" with a tone of realization.

"Jo" is not only a positive reply to negative questions, similar to "si" in french, e.g. "har han ingen barn? Jo, han har mange." But it is also used for emphasis, e.g. "Der er han jo." In English as it was spoken during the 1920s through the 50s that would be like "why" in "Why there he is!" In this scene "Jo" should be translated as "Aha" or "Say" or "Hey", or "Indeed". I watch a lot of classic films so I would expect "Say" as he then continues the thoughts of the stranger discovering something that he hadn't noticed before.