Guarde o tenga los objetos de valor bajo llave.

In Spain, If someone tells you "por favor, guarde sus objetos de valor en la taquilla" means locked, as "taquillas" normally have a key. "guarde sus objetos de valor" is formal addressed to "usted" "guarda tus objetos de valor" is informal addressed to "tú"

Taquilla is more commonly the box office of a theater. "Bajo llave" conveys the message of keeping them in a safe place, if that's the case.