I saw something like this happen when I was younger (this was pre-web, in the 80s or 90s, so I unfortunately haven't been able to find any online references to it): there was a big bank heist, and the stolen banknotes were new notes which hadn't been put into circulation yet. The ranges of their serial numbers were widely distributed by the press, and for instance cashiers at supermarkets were supposed to verify whether the serial numbers of the banknotes they received matched any of these ranges (since the banknotes hadn't been put into circulation, they were treated similar to counterfeit money: they officially didn't have any value). The country's currency has changed since then (it was the hyperinflation times), so that whole banknote series is no longer valid nowadays.

If someone pays you in cash, do you really look up the serial numbers of all the bills you receive?

Actually, some people do.

There are tools like EuroBillTracker (https://en.eurobilltracker.com/) where you can enter the serial number of bills that you received and can watch them travel around the world. If someone else tracks them as well, that is.

I entered 31 serial numbers over the last 15 years. Those haven't been seen again so far. I guess it's not the right kind of game for me... :)

I played with Where's George? for a few minutes, then got bored. None of these things seem to have enough adoption to make it interesting.

If there were money in it, someone would throw OCR at the problem. Say, attach prizes to certain bills, or finding certain patterns of bills (say, two bills whose serial numbers are mathematically related a certain way).

If a government wanted to encourage spending, they could turn the lottery inside out by offering payouts on cash being spent by everyone. You take a picture of your money and if the serial number is today's lucky winner you get $1MM. Though maybe that would encourage hoarding instead? Cobra effect perhaps?

Euro coin tracking was a bit popular in the early days of the euro, as it showed how coins moved around Europe (you can’t identify individual coins, but each country has its own coins that can be used throughout the euro zone. Spanish and Italian coins move to Germany and the Benelux faster in summer than in winter, for example)

You can still play that game with new coins, but it’s less f visible now, as most coins are old and those already are well dispersed throughout the euro zone (and, of course, more and more people pay with a card)

No, but eventually they end up at the bank or similar and tracing process starts. One banknote probably won't tell you much, but if thieves are spending constantly you will find many, and you only need to get lucky once.

This isn't so far fetched. Cash registers already have Internet connections and can be outfitted with cameras to record all bills going in, except maybe those inserted in a stack. And ATMs might already be recording serial numbers of dispensed bills. Essentially they might/could do with currency what they do with license plates, which is track where everything goes and at what times.

Like 0 cash registers I have ever seen employ this.

Truth is, most cash these days probably only has one or two transactions between bank withdrawal and bank deposit.

If the banks track serial numbers, they could probably build a fairly complete picture of what kind of transactions are going on. With the vast amount of data, you could probably fill in a lot of the gaps.

It's a feature on some money counters, which seem like something banks might use at the end of the day to check the balances in the drawers.

  ATMs might already be recording serial numbers

I've been wondering about this forever.

Say John gets bills from the ATM then pays dealer Dylan for weed. Dylan then buys a beer at a bar.

If Dylan gets convicted, and banks collaborate with the police, then John gets subjected to - at least - parallel reconstruction.

Plausible deniability wrecks that entire thesis (someone existing between John and Dylan such as a gas station)

Gas station isn't someone. It's an endpoint, as the bar in my story. If the gas station cashier takes from the register to get a bier, you're right. If not, my thesis still holds I think.

What if I pay with 100 Euro bank note and get back "tainted" 50 Euro note and then pay in bar? Bank notes are not just collected and sent back to bank.

You're right but the higher the denomination, the smaller the chance it'll serve as change.

Doesn’t matter, plausible deniability means a reasonable person might spend it. Without knowing who’s next, you may have circumstantial evidence but that’s not enough to hold up in court.

You don’t punish someone based on chance.

No, but I might deposit that money in the bank, and the bank could check those serial numbers.

The kidnapper of https://en.wikipedia.org/wiki/Gerrit_Jan_Heijn was caught because a teller recognized a bill number that he had spent

And keep a list of all serial numbers of stolen money to check against.

This sounds like an use case for blockchain.

Nah

Or... just keep a database with an API you can look up.

There is no need for it to be trustless and distributed.

I don’t think the “confiscate” part is categorically true.

If the police arrest suspected criminals and find money that can be traced back to criminal activity, they’ll confiscate it (permanently only after a trial). “Traced back” typically need not even involve checking serial numbers. If you’ve a lot of cash, but no regular job that explains how you could have that much money or why you would keep it in cash, that’s enough to confiscate it.

If, on the other hand, they find John Doe in possession of a banknote that was paid out in a ransom or stolen from a bank, and cannot link John Doe to any crime, John Doe can keep the money (they’ll ask him whether he knows where he got it and may be able to force him to exchange it for untainted money, but that’s it)

(Counterfeit money is different. If the police finds you in possession of it they’ll confiscate it, even if they know that you’re a victim)

Unfortunately due to the abuse of civil forfeiture the "permanently only after a trial" part isn't true these days.

Which is why thieves prefer used notes.