Easy one.... one or more FI injectors is leaking overnight into the
combustion chamber, a classic problem! have a GOOD mechanic do a fuel
pressure test, if it does't hold pressure, have the fuel rail remove, and
just watch the yellow tip of the each injector... one will leak.

http://www.benplace.com/fi_hoses/100_0006.jpg
http://www.benplace.com/fi_hoses/100_0025.jpg

Cheers, Ben
http://www.benplace.com/vw2.htm