Nothing would but if the cops thought the cases were linked (even if they are not, or not the way they think), they will go with the last & only lead, Lewis & If Lewis was known even just casually by James, the guy who lives in that Rutherglen flat. then they will automatic assume he must know more about Graham & now after 2 people, they of course going to seal it off & look. They will look everywhere at this point. They were so focused on these men that the neighbor could just vanish. The neighbor of course could be some kind of handy man or cleaner or have access to properties & that's how they got connected with all of that. The cops are just looking for any hint of a crime scene, they would be securing Buckingham Palace at this point. The hire van could be the crime scene. Some killers do that. He was pretty neat in vanishing. Even got a bag ready. Guy like that.... probably not his first rodeo. I don't think a guy like that works with anybody. But somehow he did leave that bunch hanging. Maybe they were an "easy" bunch. 2 people fighting a love tiff, Graham maybe clueless & only focused on his sports (that man was some fan), Who would have missed a busker. Males missing don't usually get that attention. At this point they really should put out an alert for that neighbor. They can easily do it asking for a witness. They do that all the time even without it being murder. This kind of guy follows every news item they make about it. I'm sure police do their best but a bit of help of the public could be welcome.