The thermostat housing is just below the coilpack, these are made of plastic which does often break causing leaks.
The water cooling system does NOT suck in any water, as it can't it's under pressure, so any leak is always outward.
A leak does lower the pressure inside the system though and these can and do run over 100C, so it needs this pressure to raise the boiling point of the coolant.
Where are you filling up to ?, they only need to be on the minimum, but some settle at UNDER the minimum, like my old one(now my dads) 60K on it's new engine coolant settles about 1 1/2" BELOW the minimum, but doesn't use a drop, I merely marked the level where it settled and it stays there constantly when checked.
The cooling system is doing what it should, water expands when it gets hot as do most liquids, hence the tank being a expansion tank, with the fans coming on the water cools and effectively shrinks dropping the level.
Just fix the leak is all that's needed.
tabetha