Have another bomb in the vicinity that did not fail and there will be absolutely no visible indication at all that something penetrated the ground.

I might be imagining this but I think that I have seen a report about some systematic effort to trace bomb dropping patterns in aerial images of the aftermath to find gaps in the pattern where something did not go off. But no idea how early or late after the war that happened (if it did happen). And those patterns will only show up for smaller bombs dropped in quantity, not for the big ones.