Do bluetit chicks fledge without the parents realising?

Yesterday my bluetit chicks were happily chirping in their nest box as I was hanging out my washing. I have been hearing them for a few days. Today at around lunchtime, one of the adults brought food, but flew in and out of the box five or six times, still with the food in its beak. It eventually flew away, so I went and listened, but the box is now silent. Have the chicks all fledged without the parents (or me) realising? Do adults normally return to empty boxes with food, or should I think the worst?