Gradual Pressure Increase in Buderus Logamax U042-24k Gas Boiler: Normal or Concerning?

Welcome. For a year now, I have been the happy owner of a studio equipped with a heating system based on the heating of water by the Buderus Logamax U042-24k gas boiler. The heating circuit is not connected to any other circuit, yet from the very beginning it observes the gradual increase of the value indicated by the pressure gauge needle. When the plant was not in use (March to mid-October this year), the pressure slowly rose from 1.5 bar to 2.5 bar. Before the winter start-up, I reduced them to about 1.6 bar, draining about 1.5 liters of water. Since then, in two weeks, its value has increased to 2.1 bar.
My questions are as follows:
- Are such gradual increases normal in a new installation (from June 2016)?
-If it is a closed circuit, the water is at the same temperature all the time, where does the pressure increase mentioned earlier, but quite significant, come from?

Buddy, if the pressure rises even when the stove is not working, you must have a damaged valve allowing water from the network.
Maybe it is not closed slightly?

In the meantime, I checked the meter reading. When the boiler pressure changed by 0.5 bar, only 0.0003 m3 "escaped" from the meter, so this is not it.
I would like to add that the boiler is under warranty, so if we fail to come up with an easy solution, the service technician will have to deal with this problem.

A colleague, a service technician will issue an invoice for such a diagnosis.
Buddy, either the water gets into the system from an external source, or you have a damaged pressure gauge.

Therefore, before calling a service technician, I prefer to spend a few moments and find out for yourself what the causes may be. As I understand all the hypotheses apart from the valve passing water from the network to the circulation, which can rather be excluded?

Edit: the gauge is 100% functional.

Water pressure increases when we heat it up or it comes from another source.
If you say that the pressure gauge and valve are ok, maybe you have a damaged diaphragm tank. This is where air under higher pressure can enter the system.

For posterity: remove the water release valve from the network, clean it, reinstall it and enjoy a working boiler. I am closing, thank the colleague above for the answers.