This was a good episode in my opinion. Let me address a couple of items that you pointed out. First of all I believe that the conversation between Columbo and the sergeant was the sergeant telling Columbo that they found the gun on top of the elevator. In fact later on Columbo mentions that to Kay. That they found it and replace it with a duplicate. And the sergeant did not want anybody else to hear that information. As you may recall he wanted to take Columbo in the production Booth to talk to him in private and Columbo refused that because he just messed up somebody's film.
The other thing that I wanted to point out is the over playing of the carnival music at the end. No the crew had taken a break from filming but yet they had that carnival music blasting for about 6 minutes at the end of the episode. Not quite as bad as the Volare from Troubled Waters but almost.