Had the car been through a rain or car wash recently?
If there were no other sources of moisture inside the vehicle, I would suspect that your A/C evaporator/heater core chamber isn't draining properly. However, I think if that were the case you would have experienced some "musty" odors from your vents prior to now.
Mk3 |╠╬╝|SGM|203a|Moonroof|17" Paint/Mach Whls|K&N 57S-4000 -snorkel|FFE RMM|Steeda STB, RSB|JBR bushings
Mk1|╠╬╝|Pitch Blk|Prem pkg|Moonroof|SVT Snorkel|SVT Susp|Steeda STS|Z-Eng RMM|Magnaflow exh