Well they've not been away but they are much more noticable at this time of the year busily stocking up and hoarding food for the winter, when food becomes scarce.
...there are lots more around cos the little babies magpies, which have kept them busy all summer are now adults..........hence there are 3 or 4 times as many to see....;-)
They certainly didn't disappear from where I live. In fact I think the youngsters have hung around after rearing as well. Along with their cousins, the crows (and grey squirrels), they seem to be multiplying their numbers every year. No wonder the smaller birds, like the common sparrows, are disappearing.