Central Hearing Leak

I’m looking for some expert opinion in a problems with my central heating system. We have a condensing combination boiler which was fitted in 2015. Whenever we have used the boiler we have always had to periodically top up the water to 1bar as it would drop every few weeks. Recently the pressure has been dropping more frequently and we have had to top up daily, then twice daily. However, over the last two days the heating would come on in the morning from 6am -8am. In the last two days , once it had cooled down I topped the pressure back to 1bar from 0 and the pressure was dropping back to 0 when the central heating had not even been used. From 1pm to 10pm it had dropped from 1bar to 0 when it had never been turned on! No water omitted from any pipes outsides either. I called the plumber out who initially thought the heat exchanger could have cracked internally because there were a few drips of water coming from it inside the boiler when he removed the condensate siphon which was full of water. To check this for certain, they topped up the water pressure to 3bar and came back 1hr later to find the pressure had dropped back to 0 but no more water had been collected from under the heat exchanger. They then emptied a bucket of water from the expansion vessel and pumped it up and told me it would appear we have a leak under the floor that we need to locate as we would have noticed a leak upstairs by now. They thought in that 1hr we could have lost around 3pints of water so the loss has to be under the floor I should also mention that there has never been any water coming from the external overflow pipe outside from the back of the boiler, and there are no visible leaks on any of the radiator fittings or pipes in the house at all. As you can imagine the thought of trying to find this fills me with dread!! Has anyone any other thoughts and suggestions on how we try to find a leak under the floor joists other than cutting holes everywhere? What methods could be used ? I saw something about filling the system with a smelly liquid to locate it? Any help appreciated.