The gauge has a limited job
A pressure reading describes the system at that moment. It does not distinguish a concealed pipe leak from a visible joint, an appliance fault or another system issue. Keep any earlier engineer findings with your notes.
Make the pattern visible
Write down the time, reading and whether heating was operating. Note recent work and visible moisture without dismantling fittings. The rate and conditions of loss help decide which checks are useful.
Focus the next investigation
Where hidden pipework remains suspect, suitable testing of available sections may narrow the search. The installation and access determine what can actually be isolated.
Explore the relevant service
General information for planning an investigation. The property, instruction and findings determine the appropriate work.