Commonly used to refer to a small bag sewn into clothing.
Siempre guardo un pañuelo en mi bolsillo por si acaso.
I always keep a handkerchief in my pocket just in case.
No olvides llevar un mandilo en tu bolsillo por si necesitas limpiarte las manos.
Don't forget to carry a handkerchief in your pocket in case you need to clean your hands.