Just as I thought everything was going well with the car

I've had some monster condensation in the car of late over the last couple of days which made me wonder. So I had a bit of an inspection tonight and the carpet in the rear passenger side is wet. It's quite a small patch, towards the centre console and nowhere else. There is no water anywhere else. It doesn't small of washer fluid or anything like that, it just smells of stale water.. Any ideas where this could be coming from ? Had a bit of a search but not really any the wiser. It has been very wet recently but the car hasn't been out much so I can't see where it could be leaking from to get a specific patch like that..