Larry,

Sounds like condensation to me.  If the rain were leaking through the slabs
that fast, you (and your car) would be in deep, uh, rubble.

The unpainted concrete must have been exactly the right (or wrong)
temperature for the moisture in the air on a rainy day (and the water on the
floor slabs to evaporate) and condense on the bare concrete.  The painted
concrete must've been slightly warmer (?) so the condensation didn't occur
there.

Needless to say, our esteemed colleagues may pronounce me to be full
of...rubble in regard to this matter.

Ralph

I've always wondered what one can tell about the temperature and humidity
from condensed breath on a cold day.  Not to mention where the 4 minutes went.