Condensation will form on cold surfaces when warm air comes into contact with them - the air cools and can hold less moisture - it may be that you are putting your car away warm (or even wet and warm) and that's how the mositure is getting into the garage. The dampness in the air may have built up over an extended period until there was enough of it to form the condensation droplets you see.
Try this - leave the garage door open for a couple of hours to let it ventilate and any residual moisture to evaporate.
When garaging your car(s) allow them to cool down for an hour or so before putting them to bed.
You might also consider ventilation bricks to get an airflow through the garage.
Good luck...
Peccavi, March 2010
