Coolant Loss Mystery?

Hi all,

Just wondering if anyone can help or advise with this please?

I am experiencing large levels of coolant loss in my '97 CSE Turbo, as in a litre a day being used.

Garage has carried out a check for leaks (smoke test), all ok
Cylinder block test: passed
Pressure test: passed.

Due to the above, I am now at a loss to explain this. Just wondering if anyone can make any suggestions where I may go next with the car please?

Thanks in advance!!