Odd coolant loss issue

[Polyal]

OK I will take it to the garage tuesday but I was interested in your thoughts before doing so. Basically I seem to be loosing around half the top coolant tank (clear plastic one) every time I let the car sit over night after a long drive.

Thing is though, while its driving it stable, ill go for a long drive and the level is the same. Temp sits on "O" and doesn't budge and its running just fine.

But eveytime I come home and then drive it the next day there is a small puddle under the car. My guess is that there is something that must expand when the car heats up, or that the pressure increases so the leak stops.

But when it cools down (overnight) then a crack or something opens as the part constricts and then water comes out.

Its not effecting the performance at all, but I bit disconcerning putting ~1L in a day.

Definitely not a HG issue as there is no rough running, no steam out the exhaust etc and the "leak" seems to be towards the front.

Any ideas?