It looks like condensation. Windscreens suffer more because they are big and near the vents.
When you are sat in your car you are breathing out moisture laden air, filling the car with it.
You get out and the car cools and the moisture condenses on whatever it touches that is cold.
The windscreen vents blow more air over the windscreen than the other windows without vents.
If your car is warmer when you stop you will have more moisture in the air.
So long story short, you're going to have condensation, warmer cars will have more, cover your vents.