1. A method for determining a customer's location in a retail store checkout line comprising: a customer entering a retail establishment carrying a mobile electronic device proprietary to the customer; the customer opening mobile self-checkout software on the mobile electronic device; the mobile electronic device sampling, after the opening, ambient noise; the mobile electronic device analyzing said ambient noise to search for sounds emanating from a POS terminal; and the mobile electronic device determining, based on the analyzing, whether the mobile electronic device is within a threshold distance to a POS terminal.