Nooooo don't leave without saying anything! If you're leaving a group always make it known I think. I have a friend who leaves without a word and my other friends bitch about him ... I think he should say he's leaving at least to let us know and not worry.
I'm fine with Au Revoirs - I alwaysss hug my friends (well female ones at least cause my straight guy buddies don't hug) when I say goodbye.
In regards to Adieus, I'm horrible at it, I cry. I get so emotionally attached to people, and when it's time to leave it's super hard. Primary school, high school, relatives from overseas, people who have passed away, TV shows that end - I cry and I hate it !!! To me, it's one of the hardest things in life.