how to find leak?

I had pressure loss needed topping up two or three times a week.Eventually, after lots of boiler services(10 year old) I decided on a new boiler, fiited ten days ago.
But my pressure keeps dropping and when fired up rises from 1.5 bar to over 2.5.
Tested all rads with tissue overnight, no signs of leaks.
Engineer who has been with me for years and is an old friend tells me to be patient, it may be air still in system.He has added a sealent and inhibitor and promises to pop back this week.Any good ideas, this is driving me mad simply monitoring it!