Yes, I also doubt it's a leak, even a tiny hole would allow for more loss per week that you are registering.
However, to be safe, try to empty your backwashing further away from the pool to avoid it soaking under the pool, this will tell you for sure, it would probably take close to a week for the ground under the pool to drain and become harder, but over the course of that time, you could always check your soil directly beside the wall at the soft spot to see if it's continually damp - this would be a sign of a leak.