[Dianna]

Yet the "code" must be meaningful to someone other than the sniper. Otherwise it's a purely private joke with no sharing the punchline.

The sniper wanted it said outloud. Knowing that people would start looking for the phrase, read the story and know the Police are fools?

[snickeroon]

Well, here's another scary shorthand possibility (LOVE the anagram site)
"A SUCKED ION ON"