The location of the next clue is written on a glass window or on a mirror. When the glass steams or mists up the clue is revealed. A good tip is to write the clue with a midly soapy finger as the writing will be slightly clearer.
This type of clue can be a lovely way to start a treasure hunt, as it could be revealed when the treasure hunter has a bath or shower and then sees the first clue written on the glass.