ABS light as well as ESP light can mean the ABS module has failed like whats happened to me, At least £350 to get it rebuilt or over 1k for a new unit if it is the ABS module, Get it scanned to find out ASAP.
My S3 started doing this about 2 months ago. By checking with vag-com, it turned out to be a dicky ABS sensor. ~£15 from your local motor factors, easy to fit the new one, but getting the old one out can be a bitch. I ended up having to use a hammer and chisel on mine!!!