Many times air flow plays a role as well. As Clint suggested, their's may have been open to the air, and/or for some reason their slab was warmer. Pretty hard to tell now.
In fact, my house in Chicago's entry vestibule had condensation just in one corner a few days ago. Four walls brick, stone floor. Dead air space where it was damp. And the garage there OFTEN will have wet areas when the ambient conditions are right.
But barring a leak (which would be obvious), condensation is your source. Especially with the warm moist air you guys had then.