I blame the Coal-Tits.

Apparently they grab a seed, dash off, hide it, return, repeat; or so I am informed. This evidently accounts for the fact that they vanish with what seems like several bodyweights of sunflowers over the day.
Add that fact to the fact that this one is in excess of  "LONG" yards away from where I stand and sprinkle the odd handful for the visiting handful of feral pigeons; who are much more interested in scoffing the lot, and fighting, and courting, than hiding any for future reference.