When a magnetic compass is not in use for a prolonged period of time, it should be:

A magnetic compass should be shielded from direct sunlight when not in use for an extended period. Exposure to sunlight can heat the compass, which may affect the liquid inside and potentially lead to inaccuracies in readings due to thermal expansion. Additionally, direct sunlight can degrade the materials of the compass over time, impacting its longevity and reliability when needed again.

While ventilation is important for preventing moisture buildup, leaving the compass uncovered might subject it to dust or other contaminants. Storing it in a damp place is counterproductive since moisture can corrode the components and lead to malfunction. Placing the compass in a magnetic free zone could be ideal in certain circumstances; however, the immediate priority when it is idle is to protect it from environmental factors like heat and sunlight. Shielding from direct sunlight helps maintain its functionality and durability the best.