Secret codes exist around the world for many reasons. The primary reasons? To decrease panic or increase privacy. The codes in the collection all fall into this category.

Thanks a bunch to Chris Baraniuk of BBC and Jamie Frater of ListVerse for their articles on Secret Codes that inspired this article. Links to their work are at the bottom of this article.

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Visual codes

Not all codes are alphanumeric. Some are visual, intended to be hidden in plain sight. As BBC Future discovered earlier this year, many banknotes feature a specific pattern of dotscalled the EURion constellation, placed there to prevent people from photocopying money. Many copiers and scanners are programmed to spot it.

And online daters might want to use their profiles to alert potential partners to the fact that they have a sexually transmitted infection, but to do so discreetly. Theres a codeword for that, too: 437737. On a telephone dialing pad with letters associated with numbers, the number spells out herpes.

Other visual codes are scrawled in the landscape around us. One surprising example is the series of signs known as hoboglyphs a collection of symbols meant to provide information to travelling workers and homeless people. Among other things, these could indicate the quality of a nearby water source, or suggest whether the occupant of a house is friendly or not.

(Known as hoboglyphs, these nondescript graffiti tags highlight safe areas, water sources and information about police between the homeless (Credit: Flickr/Everfalling/CC BY 2.0)

The Ten Codes are a list of codes used by law enforcement officers in the United States. They are available on the Internet which would make them seem inappropriate for this list, but a large number of police departments have tried to have them made illegal for distribution, so they deserve a mention. The codes were developed initially in 1937 and were expanded in 1974. The California Police use a variety of extra codes which predated the ten codes. For example, a 187 (one eighty-seven) means homicide. In the ten codes system, a 10-31 means that a crime is in progress, a 10-27-1 means homicide (the 10 is usually not said when it is a three-number sequence), and a 10-00 (ten double-zero) means officer down all patrols respond.Wikipedia has a complete list of the police codes here.

In computer support, a variety of codes can be used when referring to a customer. One of these codes has become fairly well known on the internet: PEBKAC (Problem Exists Between Keyboard And Chair) but there are a variety of others that are lesser known. One of these is used when reporting a fault which has been fixed: The fault was a PICNIC (problem in chair not in computer), or ID 10 T Error ID 10 T is, of course, IDIOT. Let us hope that you never see this noted down on your file when a serviceman is fixing your computer.

Time Check (usually taking a similar form to: Time check: the time is 12:00) can be a code in stores for a bomb alert. It alerts the staff to follow the bomb procedure, which can be to either try to locate any suspicious packages, or to prepare to get the hell out. If you hear a time check in a store, it is probably a good idea to start moving toward the exit. Surprisingly and shockingly, the majority of stores that use this code actually expect their staff to search for the bomb certainly an aspect of the job that the majority of teenaged checkout operators werent expecting when they signed up I am sure.

A code 10 in hospitals can refer to a mass casualty or serious threat (such as a bomb alert), but the majority of people experiencing a code 10 will do so for another far more common reason: a code 10 authorization is made by a merchant when he needs to call a credit card company to enquire about your card. This means that he is suspicious of you or your card and doesnt want you to know it while he gets it checked out. When the credit card company hears that they have a code 10, they will ask a series of yes/no questions to the merchant in order to find out what the situation is. This will often result in the merchant keeping your card if they believe it is safe to do so. This type of call often results in a call to law enforcement.

Doctor Brown is a code word often used in hospitals to alert security staff to a threat to personnel. If a nurse or doctor is in danger from a violent patient or non-staff member, they can page Doctor Brown to their location and the security staff will rush to their aid. In some hospitals, code silver is used to refer to a person with a weapon, and code gray can mean a violent person without a weapon. Hospitals have a huge array of various codes to describe all manner of situations. They often differ from hospital to hospital and they are usually not internationally recognized. [Image courtesy of The Adventures of Dr. McNinja]

On a ship, a code oscar means someone has gone overboard. If the ship has to maneuver erratically to handle the situation, it must also send out blasts on the signal so that other ships nearby are aware of the fact that it is about to change its course. It should be noted that ships dont have an internationally standardized set of PA signals and they can differ from place to place, but this is a fairly commonly used one. Oh and a code delta can mean that there is a biological hazard though who knows what that might be on a passenger ship. And finally, Code Alpha often means medical emergency.

Code Bravo is the code phrase for a general security alert at airports. Unlike most of the codes on this list, the code is meant to cause alarm but not through knowing what it means: when this alert is raised, all of the security agents will begin to yell Code Bravo in order to frighten the passengers this is supposed to make it easier for the agents to locate the source of the problem without interference from the general public. For those of you who travel on ships from time to time, you may like to know that Code Bravo means fire and it is the most serious alert on a ship if it burns, you either get off or burn with it. Ships also often use sound signals, such as 7 short and 1 long, meaning man the lifeboats.

Inspector Sands (or sometimes Mr Sands), is a code for fire in the United Kingdom. Obviously it would not be appropriate for the service staff of a store to announce a fire publicly, so this code is used to alert the appropriate staff to the danger without upsetting customers. The wording differs from place to place and in the Underground network a recorded Inspector Sands warning is automatically triggered by smoke detectors. In some shops you will hear the code used in a phrase such as Will inspector Sands please report to the mens changing room if the fire is in the mens changing room. It was played on a continuous loop through the underground during the July 7, 2005 bombings, and has been incorrectly described as a code word for a bomb the frequently used code for a bomb in the Underground is Mr Gravel for example, Mr Gravel is in the foyer. Mr Sands (or sometimes Mr Johnson) is also used in theaters in the case of fire. You can listen to a recording of the Underground Inspector Sands warning here.

WalMart gets its own item on this list because they have a large number of codes that are store specific. Some of their codes should not worry you for example a code 10 or a code 20 just means that there has been a dry spill or a wet spill the biggest danger this poses to you is that you might slip over. A code 300 calls for security and a code orange means there has been a chemical spill. But here are the ones you really need to worry about: Code red means there is a fire in the building get the hell out if you hear this. Worse still, a code blue means there is a bomb in the building. Exit swiftly but dont run in case they think you planted it. A code green means there is a hostage situation and a code white means there is an accident. The one you are most likely to hear is a code c which is simply a call for customer service (usually meaning that more cashiers are needed). And finally the most famous WalMart code well, its so famous it needs its own item:

Code Adam was invented by Walmart but it is now an internationally recognized alert. It means missing child. The code was first coined in 1994 in memory of Adam Walsh, a six-year old, who went missing in a Sears department store in Florida in 1981. Adam was later found murdered. The person making the announcement will state we have a code Adam, followed by a description of the missing child. As soon as the alert is heard, security staff will begin to monitor the doors and other exits. If the child is not found within 10 minutes, the police are alerted and a store search begins. Also, if the child is found in the first 10 minutes in the company of an unknown adult, the police must be called and the person detained if it is safe to do so. In 2003, the US Congress passed legislation making a Code Adam program compulsory in all federal office buildings. A similar alert is called an AMBER alert, a backronym for Americas Missing: Broadcasting Emergency Response but initially named for Amber Hagerman, a 9 year old girl who was abducted and murdered.