I doubt a law enforcement agency would bury firearms anyplace it would be legal for you to dig.
Law enforcement officers also aren't supposed to sell the drugs they seize either, yet it happens.
An interesting question. I doubt you would be able to keep them as they would be seized as evidence of a crime. (Falsifying official documents.)
I know what I would do, but the mods wouldn't like me saying it as it's not the "legal" thing to do.