Anthony Curren
The usual cause of pressure loss is a leak on a radiator this may not leave a pool of water as it evaporates wrapping paper towel around joints may show leak. Also if gauge goes above 3 bar the pressure may be lost this could be the expansion vessel or pressure relief valve if this is the case water will be seen outside near the boiler ( a short copper pipe that points back towards the wall ). If the pressure drops whilst using hot water this can indicate that the heat exchanger is faulty.
