First off, what is the mileage on the car?
Second, has the coolant been changed every 2 years or so?
Third, when you start noticing the low coolant level, were there any cooling components changed prior?
Fourth, as per above, that puddle is most likely A/C water condensation (because it points to that spot, and it was dry the next day - coolant does not dry like this), but to be 100% sure, did you run the A/C prior to parking the car?

Once you answer these questions, we could prolly go a step further.

Cheers

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.