There is a lot of foam under the carpets on most modern cars, including the V40 as you have noted. What that means is that any minor leak you notice invariably means that the foam and floor will probably be sodden. That could be a small (or larger) rainwater leak that has accumulated over a period of time. The other two possible candidates are a coolant leak, probably from thge heater matric or interior pipework, or a leak from the clutch and brake master cylinders. If you are familiar with the smell of those fluids, it might give you a clue as to the source. Look around the upper ends of the brake and clutch pedals whilst operating them by hand for any signs of fluid, and check both with an absorbent paper towel, which will readily show moisture if any is present.
Are either the cooling system or brake fluid reservoirs low? If so, top them up, mark the levels if necessary (although if you top them up to the "full" marks that shouldn't be necessary, and check repeatedly for loss of fluid. Note that the brake fluid reservoir as baffled, to separate the clutch cylinder chamber from the braking system. You will have to check carefully around the reservoir to compare fluid levels, as a loss of fluid in the clutch system won't be reflected by the level in the brake system part of the reservoir.
Apart from that, you may have a fairly hopeless task unless and until you can dry everything out and then do some more checks. That may mean removing the carpet and circulating air and heat inside the cabin for several days until the foam and floor are completely dry. If there are no obvious leaks from the brake/clutch or cooling system, rub talcum powder around the door and boot hatch seals, lay absorbent paper towel everywhere and start by pouring water around the doors, over the windscreen etc., checking continually for signs of ingress, both on the paper towel sheets and on the talc'd door seals, where any evidence of water running will be obvious in the powder film.
If the "water" feels greasy, it does suggest that it may be coolant or brake/clutch fluid. They have distinctive smells, but if you are not familiar with them it won't necessarily help. OTOH if rainwater has been laying in the foam and carpets for some time, it could also feel greasy to the touch. Leaks can be difficult to resolve, but start with the obvious locations and chacks as above, and if nothing is obvious you will have to dry everything out before going any further. You are going to have to do that at some point anyway, because damp sitting inside the car for any length of time will lead to other problems eventually.
