There are a few reasons your fermenter pressure may drop even without a known leak.

  1. Temperature drop. Pressure is directly proportional to temperature, if one lowers, so does the other!
  2. 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.
  3. 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.