There is always a risk converting an open vented heating system which will automatically top itself up to a sealed pressurised system which will not.
You may have several small leaks so small the water evaporates and there is no water damage.
Could be a small leak under the floor. Its not the installers fault its just unfortunate for you. Id get someone to look under the ground floor floor to see if they can find anything.
Its a ground floor flat so unfortunately I can't look underneath. If it is multiple very small leaks then an engineer might not find them by lifting up floorboards. Would a specialised gas testing company be the best route, or just cut my losses replace the pipe work while the floorboards are up. It could have the potential to be an ongoing repair work. Thanks