As a LEO, one must establish PC to make an arrest and the State must prove beyond a reasonable doubt that a crime was committed.
So I'm still curious how they establish PC.
Please keep in mind that a lot of California's laws are written in a manner that makes enforcement quite difficult. The legislative process that produces law tends to result in a product that mutually acceptable to enough legislators that it can be passed, it doesn't necessarily produce law that is consistent with other laws, is practically enforceable, or is well-tailored to meet its objective. I'll use California's "Zip Gun" statute as an example. "Zip Guns" are generally thought to be crudely manufactured firearms, and a crime to possess. But when you parse out the elements of California's "Zip Gun" statutes (Penal Code sections 17360 and 33600), you'll see that there is no real feasible way to prosecute a violation.
It's also important to make sure that you've got a good understanding of the legal terms and concepts involved in an arrest and prosecution:
1) The lowest standard of certainty is often called a "Hunch", "Whim or Fancy." There really isn't a specific legal term, because there is no legal standing attached to it.
2) "Reasonable Suspicion" is the next higher standard. The suspicion must be based on articulable facts, but it's a very low standard. The Supreme Court case that first recognized the standard found it present when a suspect made multiple "walk-by's" of a storefront, while looking inside. An LEO with "Reasonable Suspicion" may briefly detain the suspect to investigate, and may perform a non-invasive personal search for weapons only.
3) "Probable Cause" is the next higher standard. It is required for the making of an arrest, and the issuance of a search warrant. It is also often the key legal component when a warrantless search is conducted. Although the root of the word "Probable" is that the outcome is more likely than not, the courts have found that "Probable" means there is a "Fair Likelihood" that the believed condition actually exists. One of the most frequent mis-conceptions about this standard is that it does not exclude other lawful explanations for the condition. It's only required that the "Fair Probability" exist.
4) "Good Faith Belief That a Conviction Can be Obtained" - This is the traditional standard for a prosecutor to file criminal charges.
5) "Proof Beyond a Reasonable Doubt" - This is the standard for a criminal conviction. It does not require prove beyond all doubt. A common technique used by prosecutors in cases like your hypothetical above, is to show the lack of any plausible alternative explanation for a defendant's conduct. The best example was the conviction of mobster Al Capone. The government never proved the source of Mr. Capone's income that resulted in his conviction, it only showed that the income was not lawfully received, leading the jury to conclude the alternative.
In your example about a person being arrested at California's agricultural checkpoint (and that may not be a good example. The checkpoints are normally manned by persons who lack LEO authority, but for the sake of example, let's proceed as if they were LEOs), the observation of ammunition in vehicle would provide "Probable Cause" for an arrest. It's the "Fair Probability" thing that makes it so. While it is "
possible" that the ammunition was exported and then was being re-imported, and it is also "
possible" that the ammunition was purchased in-state between the border and the checkpoint, it remains more "
probable" that it was purchased outside the state. This is where it's important to remember that alternative explanations don't necessarily negate a finding of Probable Cause.
But to file criminal charges, the LEO has to make the next higher hurtle, that being the "Good Faith Belief" of the DDA. I don't know of any that would file charges based only on the above. Every DDA has a boss DA and the DA's are elected. They're more concerned with conviction rates than with filing rates. But once an arrest is made, there is also the ability to do post-arrest investigation. In your example, I would expect that investigation would be to document the movements of the defendant from the time of the border crossing to his/her arrival at the checkpoint.
At trial, the DDA will only present their evidence showing that the defendant is guilty as charged. The DDA ain't go gonna down the road where the defendant exported, and then re-imported, the ammunition. If that defense is going to come out to the jury, the defense is gonna have to put it on. No defense attorney wants their client to testify and I'm hard pressed to see how the defense could raise that issue without the defendant testifying. They likely outcome of that effort is that DDA will make every effort the "shred" the defendant through cross examination, and then will argue that the prosecution theory is more credible. In a criminal trial, the prosecution has no obligation to negate anticipated defenses (although it may be advantageous to do so), or to respond to defense theories. They have to carry their burden of proof, but they get to decide how they do it.
