The ground can be wet even when it hasn't rained as a result of condensation or dew.
Dew is what forms on the ground when the air cools down to it's dew point temperature, causing water vapor in the air to condense into liquid droplets on surfaces such as concrete and grass.
Dew is especially very common on clear and calm nights, when the heat radiates away from the ground and causes it to cool down.
The air contains water vapor and when the air temperature cools down to it's dew point, which is the temperature at which the air becomes saturated and can hold no more moisture, the water vapor condenses into liquid water and forms the dew.
Clear skies also allow the heat to radiate away from the surface of the Earth, causing the ground to cool down.
And calm conditions prevent warm air from mixing with the cooler air near the ground and allows dew to form more readily.
Also as the air cools down, any surface which is colder than the dew point will also collect the condensed water and result in a wet surface.
Dew is the most common reason for a ground to be wet even when it hasn't rained.
Although other factors such as high groundwater table or even a leaky water line can cause the ground to be wet without any rain.
Dew is more widespread and seasonal.