A common problem (i had it on my A6) is the brake pedal sensor. I got one from a local VW dealer for about $8 bucks... It would be a good start unless you know someone with a VAG-COM.

The ABS light will come on if the Brake Light is not activated at the right position where pressing the petal activates the brakes.

Hope that helps.

 

Normally it is the abs speed sensor in one of the 4 wheels in my experience, not saying this is the cause of it without putting your car on computer and can not tell you which one it would be. This happened to me right after changing my wheel bearings in the rear and after installing new sensors in both wheels my problem went away.