Magical residue

Magical fingerprints

Whenever a spells is cast -- or, rather, whenever mana is released into the world after being absorbed into a being's core -- it leaves behind what's called magical residue. Magical residue lingers in the air similarly to mana, except it is just waste energy; it cannot be absorbed or manipulated in any manner. Magical residue also retains the aspect of the original mana (though for something like a ritual spell, it becomes rather muddied).
Magical residue can be sensed by beings with awakened cores, as long as their core is actually opened. This is important, because magical residue lingers for a few hours as it's gradually moved into The Arcanum Circular, so it can be used to tell what type of spell was cast in the area, and depending on how much residue there is, that can be an indicator of distance from the casting point, time since the casting, or the amount of mana used in the spell.
As soon as mana does functional work in the world, it is converted into magical residue; it is effectively created the instant a spell is cast, and accrues as the same rate the spell uses mana. Magical residue will gradually spread out from the point of casting, and traces of it can be picked up by a sufficiently powerful open core from quite far away. Over time, the residue will diminish as it is absorbed into The Arcanum Circular and taken down to The Essential Plane where it is purified back into usable mana. This absorption happens at a rough rate of .012xm; for reference, that means that a full discharge of Gwen's breach gauntlet, which holds 2000mp of mana, would leave lingering magical residue for nearly 48 hours. However, a trivial spell, such as light, will barely produce residue faster than it disappears.