Sentence:  Six radiomen told how , twice on two days after the ring was nabbed , a transmitter near Moscow was heard calling , using signals , times and wavelengths specified on codes found <WSD>hidden</WSD> in cigaret lighters in Lonsdale 's apartment and the Krogers ' house and also fastened to the transmitter lid . 
Meaning of the word marked with <WSD> tags:  prevent from being seen or discovered