It's clearly how they flirt with each other: coded messages and investigating anything the other one happens to mention!

(I am now picturing the first time they go out to dinner, and it involves Biggles sending Erich a coded message that leads him to a dead drop that leads him to a balcony at the top floor of a hotel where he finds a flower to put in his buttonhole and another set of clues that ends up with him arriving at the restaurant for dinner...)