For low plums, geese and, if they can get into the orchard, sheep are the culprits, but for higher ones I'm sure it's birds. Our Victorias are long gone, and we don't have squirrels, just a selection of corvids, pigeons, and the worst culprits blackbirds (which have also removed every black and red currant, ripe Pax gooseberries, strawbs before they are quite ripe, and as many rasps as they can stuff in). The plums were nowhere near ripe, but had not fallen, just evaporated overnight.