There are a few reasons your fermenter pressure may drop even without a known leak.
- Temperature drop. Pressure is directly proportional to temperature, if one lowers, so does the other!
- Fermentation is slowing down or complete. The residual carbon dioxide in the tank is then re-absorbed by the beer, much like if you were to carbonate the beer.
- You took a sample or purged yeast or trub.
If you want to hold a fixed or minimum pressure on your conical, it's recommended you keep your CO2 tank attached to the conical.