Vestigium (pl. Vestigia) is described as an afterimage, an imprint left on physical obejcts. It can manifest as feelings, smells, tastes and sounds. Intense vestigia can be created by magic, but all living things seem to create some amount of it. Vestigia can be removed, transformed, stored and used, though exact mechanism are not clear.
Dead bodies is bad at retaining vestigia, unlike stone who retain it very well. Peter Grant, with the help of Toby, examined how long different materials retains vestigia. They found that stone keeps the vestigial the best, followed by concrete. Then comes metal (too similar to be able to differentiate between different metals) and after that wood. Flesh is the material retaining vestigia the worst.
Supernatural creatures, and people with magical abilities, seem to send out vestigia that reflect their personality or powers. For instance when Peter first met Mama Thames, he could smell salt water, coffee, bananas, diesel, fish guts and chocolate. When he met Mellissa Oswald he could hear click-clicks of legs and mandibles and feel the flicker of wings and a hot communal breath from a beehive.
Peter Grant invented the Yap-scale using Toby the dog in order to quantify vestiga/magic. The scale on based on the intensity of the raction of the magic sensitve dog. Peter assumes approx. 200 miliyaps background vestigia which can vary in magic intense environments like the Folly. Toby rations seems to be independed of the nature of the magic in question, since he has been seen to react to ghosts, wizards and fae. Stone retains vestigia the best, that's why buildings are said to have character. Metal, glass and plastics retains vestigium only relatively okay. Human bodies retain vestigia the least.