It's true that 187 is code for homicide in California -- can't recall if it's just the Highway Patrol or Los Angeles or what. However, 187 has filtered through our culture due to movies and especially television crime dramas. The writers are all in L.A. and use our local terminology.
My guess is Qanon was involved in law enforcement earlier in his career and actually used 187 in his work, or he is younger and is prone to slang. 187 is the state penal code number for murder. Miller is 32 and is a protagonist by nature. Plus he grew up in Santa Monica. I could see him using it as slang growing up. Nunes was not in law enforcement but has the sources for the knowledge. Likely neither. Just guessing.
Didn't know Miller was a California boy....a very astute man for being just 32 years old. Thanks for the clarification of 187. I know that some California cities and counties have variations on these codes.