Psychic spore
From Tardis Wiki, the free Doctor Who reference
A psychic spore was a kind of trace given off by someone's mind. Although he escaped them physically, Praska of the Aquillians was able to track the Seventh Doctor's psychic spore because it was so potent, enabling her to telepathically communicate with him. (AUDIO: Signs and Wonders)
K9 Mark II tracked Romana II on Aoris by her psycho-spore. (AUDIO: The Paradox Planet)
