Not heard of the c8 suffering from this. But can be a number of things from looking at previous A6 models such as blocked water drains allowing water in the cabin, leaking pollen filter housings, leaking sunroofs, leaking door or window seals.
You could always get a diagnostic check to see if any fault codes flag up. If you've just bought it from a dealer then they should fix any issues FOC within the last 30 days