S40 V40: V40 R-design D2 2013 Losing Coolant

[ND1998]

Hi I have been losing coolant, only when then engine is hot.
I've looked myself and can't see any obvious leaks.
I have a friend who is a mobile mechanic who suggested to bleed the system for airlocks which I did. Still showing low coolant, called the AA and they couldn't find any leaks but found an airlock and bled it. They also ran a pressure test, combustion gas test and pressure cap test test as well as checking for oil in the coolant and coolant in the oil. All passed.
Took for a drive to test and message came up again.
Decided to try some Holts Radweld and I don't think it did anything because it still showing the message.
Booked into a garage and they carried out a block test and pressure test, which both passed. Cleared an airlock and topped up coolant.
Drove back to work carpark and message came up again so I decided to take it back to the garage but it then went away so I took it back to work and topped up before I left work. Drove home and checked in the morning and sure enough level gone down again. Topped up before work today and then check on my lunch, gone down again.

I'm worried its the head gasket but its been tested by 2 different engineers.
Also, after a lot of research I am kind of convinced its the thermostat.

Not sure what to do from here Any suggestions?

[ND1998]

Me and a friend got the car on ramps and found that its dripping around the middle of the car (just under the dashboard) so I think its the heater matrix. I'll see if I can get some O rings and change them out