Low pressure - no leak - connected to cold tap?? help please?!?

I recently had a new combination boiler fitted but since 2 days after installation the pressure drops to zero frequently and I need to re-pressurise the system 2+ times a day.

The team that installed it have been back, checked their pipe work and informed me that I must have a leak somewhere in the old pipe work.

There are no obvious signs of a leak within the property and I have had a family member check all the pipes he can get to, under floorboards, etc and no leak can be found.
I have since noticed that the boiler “kicks in” for a short time when I have had the cold taps running, and putting two and two together I wondered if this could be a cause of the problem. The boiler does seem to lose pressure more quickly when the washing machine (cold water feed) has been on or if I have been watering the garden, so I wondered if it was related?

Many thanks in advance.