I had a similar event occur and I thought for sure it was a coolant leak only to find out it was blue washer fluid from my headlight washers that wash either frozen or leaking from them. That said the cardboard idea is a good one since it would have alerted me to the fact that the leak was on the very front of the car. Hope that helps.

__________________
2013 X5 xDrive3.5i Premium
2002 530I Sport Package
2001 X5 4.4i
2001 M3