Great Tit nestbox killed or fledged?

Hi All, 

I have a great tit nest box not sure how big was the brood, but yesterday I've seen one pocking out and loads of noise. In the morning tho no sound, no parents and just some feather on the lawn. I'm wondering if they've been killed or fledged? Also, I can understand one fledgling killed by a cat but not all of them (could there only been one?).  A bit sad if they were killed, also I've put a camera up to monitor a nest and see if anyone will come back.

Do you know how far away will they fly if they've fledged? 

Thanks