It may be your PRV ( Pressure Relief Valve ) has started to weep. If not that it may be a small leak on pipework. It does not take a great lose of water to drop by 1.5 so may be hard to find. if it is not possible to detect the source then it may be under flooring or in the loft space ( depending on lay out & configuration ). If the source I not readily visible then the best option is to add a system leak sealer via a radiator, this will work it's way round the system coating the inside of the pipework. If the leak is small enough this should solve the problem but can not be guaranteed.