LINEA 24 HELP NEEDED?

My linea 24 boiler pressure should run in the blue at 1 to 1.5 But the pressure is building up to 3 I bleed of at a radiator to get it back down but after about 12 hours it goes back up .The heating is not on.Had one answer from Steve,He says he thinks it is leaking via cold water loop top up if so how would you go about checking. Or any other suggestions thanks