To faire un petit coucou is to say hello. Odile, who is alas no longer with us, used to say that when she came on the Forum (especially on Frenchie's "French" thread).
I think it's a bit better than when we say "Coooeeee!" in English. That always sounds a bit irritating to my ear. And my natural response (which I suppress, of course!) is What do you want now?
Apprendre une langue, c'est faire un voyage différent chaque jour.
from Fle pour les curieux