Low Oil

I bought a 2002 Volkswagen Jetta 2.0L about a month ago. Its in exelent condition! Had 57,000kms on it when I bought it. The only thing that I've had trouble with is the driver door window. (apparently its a common problem?) About a week after I bought the car the low oil warning came on. I topped it up with some fresh oil. Took 2L I belive it was.

Now its about a month and a half or so since I topped it up and its low on oil again. I am putting it in the garage tomorrow for an oil change and winter tire change over. But my question is what do you think could cause the oil level to drop? I am not fimilar with the volkswagens so I have no idea if there are any common problems with them.

Thanks a million for your help!
Mark