Echoes are the figments of a Soul left behind whenever the Sophont is an imprint left behind by a person's impression of the environment and their stray thoughts. Most echoes are harmless, and fade in a matter of minutes, but as corroborating or stronger impressions build up then Echoes will have longer lifespans and more complex interactions.
With the knowledge of meta comes the need to control and maintain its usage. One of the more dangerous and constant threats of meta are the Auric Echos and imprints left by the general population. These Auric Echos are snapshots of emotion left on an area by someone witnessing it. For example, a dark alley in a city leaves people feeling nervous when they see it. This means that their ideas and emotions imprint on that alley. If a Auric mage were to divine that alley, they would see the nervousness of all the people who passed.
These Echos if not controlled can grow more and more powerful. Each imprint left holds a portion of meta, and when enough comes together it can lead to very dangerous meta anomalies growing in power. Let us say the before mentioned alley has been walked past by tens of thousands of people. The combined echos and imprints from these people could cause a dangerous shade to arise from the power. These shades can then move around, causing massive waves of nervousness in everyone it passes.
You could only imagine what could happen if a max-security prisons imprints and echos were left to their own devices.