Residual Information

Residual Information (残留情報) refers to traces of information left behind in objects and surroundings by events and phenomena which occurred in their vicinity in the past. There are numerous causes of residual information and methods of use and elimination, on both Magic and Science Sides.

Principles
Most residual thoughts tend to break down on their own in time.

Science
There are several esper abilities known to make use of residual information. Types of Psychometry and the equivalent function of Mental Out make use of residual information and residual memories. Dark Matter is capable of extracting residual thoughts by running across the surface in a location where an event transpired.

Magic
In Rosenthal-style Necromancy, the residual thoughts of the deceased are used as a key code in order to slip an artificial soul into the corpse, and can take shape outside of the body, assuming a dangerous form if not properly controlled.

Forensic Cleaning
Forensic cleaning (特殊淸掃) describes cleaning methods used to remove traces of an incident, such as human remains after a fatal one, from the scene of the incident.

Outside of law enforcement and forensic investigation, it is offered as a service for real estate owners to remove all traces of any corpses found in an apartment or room, including stains on the floor, not just to remove biological hazards but also to prevent the property from being devalued. It apparently doesn't require a special license and is done by cleaning professionals for an extra fee.

However, forensic cleaning is also used by magic side organizations, such as Necessarius, to erase evidence of magic-related activities and incidents, such as battles between magicians, before they can be discovered and traced by other parties, both magical and non-magical. Given its nature, magical forensic cleaning must cover various unseen areas as well as the regular ones. It also includes certain dangers, especially if the cleaner is not familiar with the parties involved, with the possibility of remaining traps set up during a magical battle and sometimes the risk of a corpse getting up again.

When carrying out magical forensic cleaning, techniques which reveal residual information in an area (and could also be used by others to reveal the evidence if it is not eliminated) are used to identify what needs to be cleaned up, ideally doing it before the short-term memory of the event becomes long-term memory and soaks into the location. Residual thoughts are often top priority, with the goal being to eliminate a readable impression of what happened and who did it.

One method of eliminating residual information from bloodstains is through the use of artificial 'human blood'. A mixture with a similar composition (such as 92% water, 6% protein, with salt and other minerals for flavor, and transparent without red or white blood cells) is prepared, a cleaning implement such as a mop is dipped in the mixture and then it is used to wipe the bloodstains before proceeding with the actual cleaning. The method is described as being like adding a decoy stamp over an addressee's name on a letter so that the personal information is no longer readable, such that even if someone realizes that something has been cleaned, they won't be able to fully recreate what has been cleaned up.

Specific methods are applied to eliminate specific pieces of evidence, depending on the nature of the evidence. For example, when the nuns of the Agnese Forces were clean up after a mock battle between Stiyl Magnus and Isabella Theism as part of their training, two types of evidence they had to dispose of (aside from a fake corpse) included zombie powder and a large number of rune cards. The Haitian toxin was decontaminated with heat of 1000 degrees or higher, which was not enough to melt the stone walls in the vicinity. As the same method would not work on the flame-resistant cards and manually removing them all would take too much time, the nuns planned to deal with them all at once with something made from nitrogen oxide or sulfur dioxide, derived from the acid rain threat to runes originally carved into stone.

There are varying styles of forensic cleaning used by different organizations, with certain materials and variant techniques being used by certain groups - for example, Roman Catholic-style forensic cleaning makes use of holy water with salt in it, an inclusion not made by other churches.

As there is a normal amount of background information left by naturally-occurring magic, intentional and unintentional, cleaning up too much or everything can lead to magic-related people noticing a site being too clean and being alerted to the fact that magical forensic cleaning has been used to hide something.

Toaru Majutsu no Index SS: Agnese's Magic Side Work Experience
As part of their training for Necessarius, Agnese Sanctis, Lucia and Angelene had perform magical forensic cleaning to eliminate the residual information and other trace evidence left after a mock battle between Stiyl Magnus and Isabella Theism in Barcelona, Spain. The day after, they noticed signs of forensic cleaning at a restaurant, owned by a Haitian immigrant whose wife had supposedly been killed by a curse, and performed their own investigation,  eventually discovering that a curse-fearing local priest had cleaned the site to make it look like something had happened there and frame the restaurant owner to get rid of him.

During the Anglican attack on the Guild's base on, when Monica destroyed the Guild's records, immolating herself in the process, Agnese used the residual thoughts on Monica's burning clothing (which had the same 'cause of death', being destroyed in the same fire), via a spell derived from St. Vincent, to transcribe and recreate the destroyed spell book. Afterwards, when investigating a destroyed statue which might hold a clue to the identity of Guild leader Tu, Orsola Aquinas offered to contact someone to the remains of the statue so they could dive into the residual thoughts, reasoning that it should be coated in the owner's thoughts or desire to conceal through its destruction, though Agnese suspected that there might be a trap (such as one where investigating the residual thoughts would drawn the mind into a dream world) as Tu would have known someone would investigate them. Ultimately, they used a different method after noticing signs that the statue was stolen.

Magic God Othinus Arc
Hel was able to draw out 'causes of death' through the residual information of the dead in her surroundings.

Necromancer Arc
When Esther Rosenthal was inserting Huotou into Hitokawa Hasami's body, the residual thoughts in the corpse took the form of a floating brain. When Accelerator touched it, Hitokawa's memories flooded into his mind.