GR translation: you checked under the barracks lately

[Mike Jing]

V595.2-7, P605.19-24   “Solid. Hey, you checked under the barracks lately?”
Seems the CBI returnees get together to play marbles with opium balls. You
can pick up hundreds if you’re any good. Corpsman Krypton pockets his money
and leaves Bodine flexing his thumb and thinking about it, moves on copping
feels, pausing to drink from a shell case of grain alcohol and grapefruit
juice, whilst dealing the odd tablet of codeine.

What does "check under the barracks" mean here?