David Evans has an interesting article on
drifting.
He cited a recent study showing that
Distance between colonies in an apiary was the major factor that influenced drifting and ~17% of tested workers had drifted (with a third to half of these being apparently unrelated to other colonies in the test apiary).
He followed this up with another study that monitored varroa numbers in colonies continuously treated with Apistan. Any mites found were obviously introduced by bees from other colonies:
In one summer an average of 1415 and 1001 mites were introduced to each of the seven ‘recipient’ colonies in two separate apiaries.