Charlie wasn't exactly an Auror, he didn't have a flare for investigation and things like that, but really you would have had to have been blind not to see that Louis was hiding something from the way he rushed to assure Charlie that he wasn't interested in talking to Callum.
"You might have run into him?" He asked, and Charlie's mind put the pieces together while he was still speaking. "You didn't." And maybe Louis wouldn't have, but from the way Callum was behaving recently he certainly would have.