Several nuclear weapons have been lost over the decades. There's one off the shore of Georgia that's never been found, one in Puget Sound, and a couple more. If you count submarines, there's also the wreckage of the Scorpion which contains two unrecovered nuclear warheads.
also the nuke loss in canada could never have been detonated. it was just a bunch of uranium that was dropped.
cant really compare with a fully armed 4 megaton nuke that fell out of a plane. for reference, Hiroshima yeild was 21 kT.
that's 21,000 vs 4,000,000