I don't know what the tool is called but it used when removing the valve stem from a rim when replacing a tire. Anyway, this tool also can tighten/loosen the valve within a stem. Your valve may be loose and is leaking air. Have this tighten.

Also, are all tires going down by the same amount? If so, I doubt you have puncture, it may be due to outside temp changes. If not, you have a small leak (on single tire), whether it's from a puncture or a loose valve is now the question.

Hope that helps.