"I noticed something..."
At the end Gosling's character said "I noticed something - I noticed you and the detective had the exact same gun."
He's a district attorney - he sees police weapons ALL THE TIME. And they're standard issue. So when he first saw Hopkins' gun, the very first thing he should have thought was "he used a standard issue police weapon?" Sure it's a popular, common gun for exactly that reason, but it should still occur to him.
To this day when I see an M1911 Colt my first thought is "military issue" even though the military has used the Beretta since 1990.
--
Philo's Law: To learn from your mistakes, you have to realize you're making mistakes.