My understanding is that the pressure test is conducted over a 10-15 minute period. Normally you should see either no loss or at most a very slight loss of pressure (say <5lbs) or so during this period.
If you are testing over a 24 hour period, all bets are off. There are external variables that can affect the pressure, notably temperature fluctuations. And you have to maintain the same environmental conditions when testing, such as keeping the entire system in shade. I once had a precipitous INCREASE in pressure when the boat swung on the mooring, allowing direct summer sunlight to shine in the propane locker and heat up the hoses and other hardware. The opposite can happen if it's in sunlight to begin with and then moves to the shade.
I would not place much emphasis on pressure drops observed over a 24 hour period. If you follow the standard guidance and still have pressure loss, then by all means keep searching for the cause.