Saw a news program some years ago, so the details may be off a little. There was an I-10 corridor through the NOLA area where local jurisdictions where finding "excuses" to stop folks. Once they did, they found excuses to confiscate their cash (could be drug money). If any one howled, they could end up in jail. The "principle" of confiscating "suspect" money has been upheld by the USASC IIRC. Once money is seized, it is pretty much the individual's problem to "prove" that the money was obtained legally and was not intended to be used for illegal purposes. I have no idea what threshold is in place ($100?, $1000, $2000, etc.) but the principle HAS BEEN accepted IIRC.
Imagine being 2000 miles from home and a LEO pulls you over for "weaving" (this was the favorite stop method in NOLA IIRC - even though the "weaving" was BETWEEN the traffic lines.) The LEO then searched the car and the person (or else asked about money, etc.) eventually seizing the cash and leaving the person with no recourse but to fight in court (paying costs of course) or just going on his way without his cash. Eventually, THIS particular scam got so bad that it made the news and the police organizations were forced to stop it (I believe there was a hidden camera/news crew involved.) In any case, the law itself hasn't changed, but LEO organizations may have become a bit more discreet (until the next time.) Not indicting all LEOs or suggesting that this is a "usual" practice throughout the world. I'm simply highlighting my tag line. It might be a good idea to have a "tool" to use against drug mules and dealers moving large quantities of cash. But, anything that can be used against a criminal can be used against the honest person. I personally would like to have enough cash available to "buy" a new transmission if I get stranded in Lizard Lick or some place where they don't take plastic, etc.
Once again, I don't think this is a typical situation, but OP's experience makes me think either he was targeted legitimately (whatever that is) or for some "evil" intent. Who knows. BUT, in any case, review my tag line one more time as YMMV.