The chances are, though, that if you find these after a party, they aren't quite what they seem. If you happen to find a box you will discover that they are not CO2 cartridges, but intended for 'cream chargers' that are used to produce catering quantities of squirty cream. And the gas in the cylinder is not CO2 but N2O - nitrous oxide - commonly known as laughing gas. This is, it seems, the latest party and nightclub thrill.
The good news is that the gas is not illegal, and if used properly is less dangerous than many drugs. But it's not all good news.
The use of nitrous oxide for social entertainment goes back to the early days of the discovery of the gas. Although it soon became a useful anaesthetic, from very early days it was also a recreational drug, with records of it use going back to 1799. There was a time when laughing gas parties where popular, where groups of people would take turns to sniff the gas and to collapse on the floor from dizziness or in fits of giggles and unseemly laughter, an abandonment of propriety that must have seemed particularly thrilling in those often stuffy times.
This use seems to have primarily died out in the twentieth century, except amongst doctors, nurses and dentists, who have always been rumoured to misuse the stuff - but now it's back big time, thanks to these little cylinders, intended to get that cream a-foaming.
Is it a good thing? Probably not. There has been at least one recent death due to nitrous oxide inhilation. It's not that it's poisonous per se, but if you breathe too much of the stuff, you aren't breathing oxygen and you asphyxiate. It is apparently psychologically addictive - meaning addictive in the sense that theme park rides or cheeseburgers are addictive, as opposed to a chemical addiction. It's the experience that is addictive. And crucially a relatively small amount of the gas can render the user incapable or semi-conscious - not an ideal situation in a club, out on the street, or particularly if cars are involved.
There may also be longer term physical damage caused by use of the gas, as its mechanism of action isn't fully understood.
If you find laughing gas cylinders after a party, it's not a matter for panic. It's not illegal, and it's unlikely to produce as bad a result as over-consumption of alcohol. We tend to have a knee-jerk reaction to all drugs that aren't caffeine or alcohol (or if you've had a certain lifestyle, which I haven't, cannabis), but in the case of N2O, this is probably wrong. Nonetheless, it's another potential way to get into trouble, which as a parent is not something I can cheer about.