One of the most frequent conditions discovered during home inspections is windows with broken seals. Windows comes in two main types, single pane and double pane. The double pane type has better insulation due to the two panes of glass that contain a gap between the two pieces of glass giving insulation value. When the gas (sometimes argon) between the panes of glass escapes it begins to allow regular air to enter that space.
Due to expansion and contraction from temperature changes the air will pump in and out of the space. Over time the air passing in and out of that air gap leaves behind moisture and stains. Whether moisture is visible depends upon the moisture level and temperature. Sometimes condensation is easily visible and at other times there are no indications.
Having a broken seal is almost inevitable when you have double pane glass windows. The fix is to have the pane of glass replaced. There is no need to replace the entire window, frame and all. There is another way of repair. It involves drilling holes in the glass. Cleaning the interior, drying it out and then placing tiny one way valves over the drilled holes to help create a vacuum eventually. This is the less expensive method of repair. It also does not provide as much insulation value as new glass with argon gas between the panes of glass.