Had it on my old B4 as well, can't remember what it was exactly, someone told me before I've forgotten. Actually might have been to do with the air con, think there is a pipe that acts as a drain for condensation on the passenger side directly behind the glove box that can be blocked from road dirt. Nothing to worry about though if it is that but just in case, I would check to ensure the coolant system is bled properly, never heard of the heater matrix being a problem area while bleeding though.