His family had been kidnapped on Friday. On that same day, he had gone around, trying to look for them. Somehow, he had noticed one of the gangsters that he had seen together with Harry, and followed him here.
It was just by coincidence that he overheard the gangsters talking. They were talking about the two people that they had kidnapped, and were being kept at a basement.
When he thought of that, he immediately looked for a person who was living in this neighborhood. He questioned him about the layout of this bar. With that, he got information about the bar.
But just to make sure, he went ahead and asked yet another person. The information that he got was more or less the same. That was the way that he knew that there was actually a basement inside this bar.