Lumberjack is right, heat and cooling cycles can cause the water to condense out of the air and contaminate the oil.
Water in the oil is bad, period. It can react with other contaminates and cause acids to form, thin out the oil and cause mechanical wear, freeze in filters in cold temps causing the bypass to open allowing unfiltered oil to circulate, and cause corrosion.
Look for any low spots that may pool water and snow. Sometimes snow can pile up allowing water to reach an entry it would not normally be able to reach.