Pressure loss in pressurized heating system?

I have a gas boiler which feeds a pressurized heating system and pressurized hot water tank with no header tanks involved.
There is a pressure guage that displays pressure in the heating system. This drops in pressure gradually, and can be repressurized by small valves to let more water in.
I am giving it 5-10 seconds open every 2-3 days which my plumber estimates is 2-3 pints. I have no sign of leaks at all! Where is it going? No sign from pressure relief overflow, no sign from any pipes. I have only about 1 metre of pipe under downstairs floor boards so easy to check. All other pipes would flood carpet or tiles - no sign at all. When pressure drops I get banging from the pipes when run downstairs shower in the morning as pipes heat up (presumably just air in system replacing water lost).