A Mythos (plural Mythoi) is an echo of an entity or a thing that existed in one of the worlds beyond the veil of the mist, and crossed the boundary to attached itself to a person, place, or object. A mythos can attach itself to an individual and alter them physically without the individual’s awareness. Due to the effects of the mist, the mythoi are usually forced to fade away if an awakening does not occur.
A mythos in a sleepwalker cannot make itself known, but it is able to physically alter the sleepwalker’s physiology in a minor ways such as sun sensitivity to a sleepwalker with a vampire mythos. Signs of a sleepwalker being a potential shifter will show in effects that sleepwalkers might consider skin conditions or some other medical deformity. A potential rift would be a lot harder to spot.