Bias boiler loosing pressure

[Farmelectrics]

Hello chaps I'm new to the plumbing forumn usually use electricians one I'm a spark by trade but currently working as multi skilled engineer for a large retail group any way could some one explain why my boiler is loosing a bit of pressure I've always been told that the pressure should be between 1 to 2 bar but I've noticed last couple of weeks it's dropping below 1bar so I've topped it back up I've looked at all the rads to see if I have a leak some were but can't find leak any were any thoughts

 

[Alpha man]

Could be leaking from the relief valve outside?, expansion vessel empty?

 

[Farmelectrics]

thanks alpha have checked its not please re assure me its nothing to worry about is it