What is it Selwin seeks to tell Frank at Rusalochka's, but is hesitant to do so in front of Kuriatin?

Selwyn has passed by Reid Press and seen a light on where it should be off. He seeks to inform Frank, but is hesitant to do so in front of Kuriatin in the crowded tearoom. He waits until Frank exits the facility and stops him at the door to relay the information.