Baby blue tit found

We have found a baby blue tit in my truck, I think there was a nest as we heard chicks a couple of days ago but now nothing, I rang the vet who said just to leave it as the parents might be near by. We have left it all day and it’s still there and no sign of parents. Question is what to do? And what to feed it? It is fluffy and almost flying so I presume it thought it was ready ?