If the temperature is lower for the 2nd measurement that will account for part of the voltage loss. You probably don't have a short circuit. What you're suspecting is often called a "phantom load". IMO the easiest way to start looking is to have everything you know about shut off, then one-by-one pull the 12 volt fuses and bridge the fuse holder contacts with an ammeter. A multimeter with a 10 amp range is good for this. This tells you which circuits are pulling power, after which narrowing it down further can get more complicated. Measuring battery SOC with a voltage measurement is approximate at best, and requires interpretation based on the the recent charging, load, and temperature situation.
