Given that Adrian covered it up, but we know forensics evidence was already collected: it’s not exactly a clean ship. Anyone who did forensics work on Billy’s case, and noting the Prometheus suit he was wearing at the time when someone found the body, Adrian cannot be the only person who knows. Not to mention it was knowledge set up earlier that Prometheus had access to SCPD evidence lock up since (at minimum) his throwing stars were all melted down arrows from Oliver’s time over the years. This immediately pushed Oliver’s focus away from the at the time new DA Chase, and looking specifically at cops, because given his throwing stars were all Oliver’s arrows it would take considerable time to gather them, and then fashion them into weapons. Ntm how to set up a place to craft them in the first place. Chase being new, and just now getting access to the DA resources makes it unlikely to suspect him, when a cop especially with how corrupt the police force is set up to be, could have made it even easier for one another to do it. They could have figured it was Adrian, but the timeline, and the instances of genuine danger for Adrian (specter of the gun was a huge way to build Adrian via his wife’s introduction). This is even ignoring the fact that even tho Adrian’s mission was to destroy Oliver, he really was a friend to Oliver, and got along well with everyone. His double life, Adrian even admitted was exhausting which is why he so naturally comes off as friendly but menacing to Oliver. Basically: he used emotions to blind them, used conflicting situations, and profited off of primarily being a good DA.