If you're sure there's a leak in the cabin, it might be worth removing the lower panels around the transmission tunnel and feeling for any dampness, to make sure. If there's a leak it will be around the tap usually (the seal leaks and coolant drips out... but the tap is fairly easy to change).
If you really can't find anything wet inside the cabin then the leak could be in the heater pipes inside the engine compartment and the antifreeze smell transmitting through the air vents. If there's a leak, the antifreeze will leave blue or pink crystals where it leaked.
The heater matrix under the dashboard is usually impossible to remove with the dashboard in place... but I never did a Stilo. You will have to remove at least the lower part.
If you have no luck finding any leak, you could try some radiator sealant (like RadWeld). It's designed to fix a cracked radiator but it's useful to seal any minor leaks.
Once you have used it and left it to do what it does, replace the antifreeze with fresh stuff so that that coolant doesn't have the radiator repairer stuff in it. Otherwise it can sit around in the water channels and fur them up. It doesn't do any harm.. but once it's sealed any gaps, you don't need it in there.
Ralf S.