Bluetits nesting - fledged or abandoned?


I had bluetits nesting for the 1st time this year, in a birdbox at the end of my garden. They were successful and feeding young ones for at least 2 weeks, maybe more, as I did not even know they had nested until I heard the chicks.

One morning I saw the adults, agitated by a great tit, i have not seen the adults since that morning and I cannot hear anything from the box. I'm hoping they fledged, but expected I might see the fledglings around the garden for a day or so? 

Is it possible the adults abandoned them at this late stage? When would it be safe to check the box?