Coolant level

In May I noticed that my coolant level on my Passat TDI estate was just below the MIN level when cold so I filled it upto the MAX level. Yesterday I noticed that the level is back down to MIN again (when cold). When the car was serviced in June I got the system pressure tested and the VW sepcialist could not find any problems and no leaks !

Does anyone know what could be happening ?

Thanks.

It could be a leak, so small that it dries as quickly as it comes out.
Can you smell hot coolant in the engine compartment after a long run and you've just arrived? You can get a UV dye to add to the coolant and then go over the engine with a UV light in the dark and the will show up if the leak is small (such as from waterpump)