If it's leaking internally in the cooler it just gets blown out the exhaust, so not much of an issue. Also if no sign of a leak it can (and most common I think) be the water pump O ring, which is hard to check as the amount it's leaking when the engine is hot just evaporates. If topping it up every few weeks causes you no issues then I suggest waiting until you get the cambelt changed and getting the water pump done then too, as is the norm. Just make sure you're adding G13 along with the water else you'll slowly dilute it and loose the protection.
Of the 3x Mk5 TDIs I know, all 3 have had water issues, my friends 140 BKD 2004 was the water pump O ring, my other friends 170 BMN 2007 is leaking the same as yours now (cause not yet known) and mine 140 BKD 2006 was a head/gasket issue pushing exhaust gasses and water out the filler bottle.
You can get a coolant dye/black light that is great for finding a leak, as the dye stays when the water evaporates, if it's bothering you and you want to find it.
Good luck
Stu