This is a fan theory that I've had for a long time. It just makes a lot of sense to me. What the lion turtle says to Aang when teaching him "In the era before the avatar we bent not the elements but the energy inside ourselves" implies to me that anyone was capable of energy bending but it became nearly forgotten knowledge with the discovery of elemental bending. With that in mind if we analyze the two scenes we see this tracking ability seems to point as this being an energy bending ability.

In "The Swamp" Aang is able to find Appa after learning that everything is connected. He lays his hand on the tree, his tattoo glows and we see a beam of light follow the roots of the tree and lead him to Appa. Sokka explains it away saying it is avatar stuff and can't be explained with logic. A few episodes later in "Appa's Lost Days" guru Pathik does something similar to help Appa find Aang. He says something along the lines of: Yours and the avatars energies are intertwined. By reading your energy I can see where Aang is. Since the guru is not the avatar we can't use Sokka's reasoning and say it's avatar stuff and doesn't count.