The "debt" is "stored" both in ourselves — not only as consequences of actions but also the tendency to repeat them — and in others, not only as the consequences, but the tendency to imitate them, thus returning the action upon others. (We treat someone rudely, and, now irritated, they in turn imitate that action, and treat others rudely, too. Naturally, this spreads the original action dramatically. This is action and consequence in motion).
This is not like we think of "storage" as objects or written lists in a ledger. Rather, it is storage like everything that is "stored" in your DNA. It is what makes what you experience.
The mathematical values of our actions and consequences stay with us until it is all resolved. Death and life and just alternations in the flow of those forces. As explained in the course, from the point of view of those values, the "betweens" (Bardos) of life and death are just like the others, "sleep" and "waking state." They are just modifications of perception. The flow of values continues, modified by any added actions.
Actions still produce consequences even if "no one knows about the actions we commit. Furthermore, it does not matter if you are aware of the consequences or not: they still happen.
All of this is discussed in some detail in this book:
http://gnosticteachings.org/books-by-other-authors/karma-is-negotiable.html