Incidentally

apropos

(by the way — see also by the way)

im Übrigen

(by the way — see also by the way)

nebenbei

(in an incidental manner)

nebenbei gesagt

(by the way — see also by the way)

zufällig

(without looking for something/somebody)

zufälligerweise

(without looking for something/somebody)

übrigens

(by the way — see also by the way)