The end?.. 

Today the tit left the nest as she usually do at times, but has never came back... There were nothing unusual with the tit or outside.
We called her Principessa ("princess" in Italian) cause she used to tide herself in the nest so often.
Home nothing bad happened with Principessa, maybe she just decided to leave the nest for some reason (were too hungry or eggs had not hatched after 16 days etc). We still don't know...