From my experience I have found three distinct things that can be called the soul, and I'm not referring to the Freudian concept. Each has its own purpose and function in consciousness, though the separation between the three is an artificial one for the purposes of discussion, much as the separation of the proton and electron that make up a neutron is ultimately artificial.
The first piece is the energetic body. This is the "life force," the energy that is in Japanese martial arts called Ki, in Chinese medicine as Chi, that flow along the meridians known in both Chinese medicine and the Ayurvedic traditions, or in science were discovered by Candace Pert as the peptide flows (her work, publicized and simplified for a general audience, is detailed in The Molecules of Emotion; she was featured in What the Bleep Do We Know?). Interestingly, Dr. Pert overlaid the three systems and found that they seemed to be alternate ways of describing the same phenomenon. As my knowledge of the Ayurvedic and Chinese traditions is shallow, I prefer to call it the energetic body, and the energy flows and centers that run throughout the body. With death and decay (or consumption), this energy dissipates and is reabsorbed into the IS (nature, for lack of a better term) and recycled. You are what you eat. ;) This is the horse and carriage, inseparable from the body.
The second piece is the part that makes you a distinct entity, the part that makes me Jaelle and my cat Candy - it's the personality. I'm deliberately NOT using the term ego here because of the entanglements there. It is the portion that has the enforced separation of entities, but it is also the portion that absorbs experiences, learns, lives, and ultimately dies. It is the driver of the carriage, and the portion that we see when we speak to spirits of the dearly departed. I have yet to not meet one in the land of the dead when I go there, but whether they ultimately dissipate into the structure of the otherworlds or not until called upon, I am not certain.
The third and last piece is the divine spark, the part that is one with everything, the part that is aware of that Unity and attempts to guide the personality to his perfect place. Some call this the "higher self," others call it a person's "genius" - again, there's many different references to it in other cultures, but not being of those cultures I call it the Divine Spark. Upon death this part "returns" (for lack of a better term) to the One (from which it never separated in the first place, but if we're speaking of ourselves as separate entities then for sake of analogy it must be referred to as so). It carries the life's mission, and the sum total of the life's experiences into the Unity of All and adds to the collective experience.
And yes, did I mention I'm an animist? I'm also a pantheist, obviously. All things contain these three; all is one. However, in order to experience life, in order to experience separation, we are artificially separated in this manner from one another into individuals. I believe it is our duty to truly be the individuals that the three segments drive us to be, for it is there that we will find meaning in life and accomplish our life's goal. So at the same time we are all one entity, and we are all separate beings, and simultaneously must strive for the ultimate achievement of both in our lives. I know this sounds paradoxical; this is where the land of poetic inference resides, and where I feel we as a species must learn that OR is not nearly as Powerful as AND.
So given this model, in my experience, most "soul retrieval" is a phenomenon of the personality interacting with the divine spark. Essentially, the divine spark cannot lose anything, so the journey is undertaken to find what the personality lost (through crisis, injury, abuse, etc). I have seen this done many ways, but I must add that this sort of thing is not something that should be undertaken for anything major without help from a psychologist or minister handy (depending on the type of thing retrieved). Once I was part of a group that had a guided meditation of going deep into the self, selecting something found there, and returning with it... and two members of the group were completely shattered by the experience, having brought back childhood trauma.
I have noted however two other kinds of "missing soul," both involving severe illness. In one instance, only the personality was gone - and once found and convinced to return, the subject came out of the illness. This has happened to me when I was extremely ill - in fact, every case I've seen of this is described as a "near death experience." The second case is new thanks to modern medicine. When the divine spark is missing what you have is often called a coma, a term I think is too broad because I've known people to come out that were supposed to be vegetables, and others to not come out despite normal brain activity. I suspect in the first case the personality is gone as well, but since after death the personality often will linger for a little before moving on it's hard to say.
So that's my personal perspective on the nature of the soul, and soul retrieval. I reserve the right to be wrong. ;)