How did Exley come to the conclusion that Dudley’s men planted guns at the suspect’s location?

Question

After the fight between Exley and Bud White over Lynn Bracken had subsided in the records room, Exley shared his opinion with Bud White that Dudley had wrongfully accused three African American men as suspects in the Night Owl case because of their past criminal records, and also that the Mexican victim girl had lied about the time the perpetrators left her:

Exley: The rape victim lied in her statement. The first guys to the Mercury coupe were Breuning and Carlisle.

Bud White: Dudley's guys.

Exley: They planted the shotguns.

Earlier in the movie, Jack and Exley learned the suspect’s location from Leonard and arrived there to find Dudley’s men already present, possibly to investigate the suspects. How does this suggest that Dudley’s men have planted shotguns at the suspect’s location?

How did Exley come to the conclusion that Dudley’s men planted guns at the suspect’s location?

Answer 1

In the source novel by James Ellroy we get a little more of Bud's thought process. The idea that the shotguns were planted comes from the assumption that the three people accused of the Nite Owl murders were in fact innocent because the motive was 'smut intrigue' not just random violence.

If they're innocent then the evidence against them must be coincidential, misunderstood or manufactured. He puts the shotguns into the latter category.

Theories on top of theories, theories that proved he had the brains to call himself a detective:

Say the Nite Owl snuffs came out of smut intrigue. That meant the niggers were innocent, the real killers planted the shotguns in Ray Coates' car--which meant that the purple Merc seen outside the Nite Owl was a coincidence--the killers couldn't have known that three spooks were recently seen discharging shotguns in Griffith Park and would rank as natural first suspects. Somehow the killers found Coates' car before the LAPD--and planted the shotguns, print-wiped. It could have happened a half dozen ways.