Hydrostatic Test

Either there is a leak in the pipe, or the size of the pipe is changing with temperature. Try a 24-h test and see if the pressure is the same at the same time the following day. If it is "near enough" the same, then there isn't a leak.

Is the gauge only going down or on both side?

may be liquid during filling process is hotter and cool down as time passes.

Or what about valve thats stops liquid to go back.is there some NRV? you should also check this.

If its trend is declining then leave it for longer period and check if it eventually go to zero? if so thats good enough to say yes there is a leakage.

I'd suspect part of the test rig... probably the isolating valve as indicated by another poster.
So....test the test rig ... simply block the outlet and pump up to pressure.

Check you have ALL the air out of the system - air is far more susceptible to temperature / pressure variation than water... pump, bleed, flush / bleed.

Not only is air in the system sensitive to temperature change, but many of the gases in air are soluble in water, especially at high pressures. So a dropping pressure can simply be trapped air dissolving into the water.

"This phenomenon is even more as the length of the pipe to be tested become shorter."

I would have expected a greater fluctuation with a bigger volume.
This indicates..... at least to me ... that it's not a trapped air or temperature related problem.

Very Good Answer

All the ways suggested i all other posts are perfectly valid. I would like to add couple of more:

If the item to be tested is small enough, test it in air conditioned room. Temperature variation effects will be avoided.

Use two isolation valves in series. The leakage backwards will be reduced drastically, by serial drops and because of statistical reasons.

If the pipe is buried shallow, the sun will heat the surrounding soil and cause the pressure to increase. Conversely, when night falls, the temperature and pressure will both decrease. As stated earlier, if you pressurize the pipe and then test it for 24 hours and the pressure does not change, you have a leakproof system. Also mentioned earlier, was the presence of air. To get an accurate pressure test, the pipe must be completely void of air. Air reacts differently to pressure than water and will keep you from getting accurate results. Always fill the line from the lowest elevation, and allow the air to escape from the highest elevation.

As you compress the fluid, it gets warm and expands. Upon reaching test pressure, no more heat is added and the fluid begins to cool and the pressure drops. Even in the absence of leaks, you will see this kind of behavior. You can minimize the effect by adding a pressure accumulator to the test set. You have already determined that a long piece of pipe will serve as an accumulator. However, if you are looking for small leaks, you will find that a gas leak detector has far greater sensitivity than a liquid leak detector.

Up to what pressure you are testing these pipes and tubes ? What type of fittings do you use ?

if there is any internal leakage, you would not know so you have to use leakproof fittings.