The windscreen leak is becoming ever more common due to the number of replaced windscreens now being fitted. I'm not sure what makes the smart windscreen harder to fit but it must be, I have heard and been contacted by countless people with a leaking windscreen.
Lift up your carpets in both footwells and feel the foam backing for damp.
Do the same for the boot carpet that covers the engine bay, also feel down the back.
If it is dry then hoorah! If it is wet then you have a Roadster,
a Fortwo with a replacement windscreen or an unusual leaking Fortwo.
Now, the chance of being in the right place at the right time to catch that drip appearing from the problem area and disappearing into the carpet is pretty shallow. Even less likely if the windscreen is leaking because it is only about 2 inches of metal before the drip can hide behind the dashboard.
Pin Pointing Leaks