Google is slowly but surely unifyning its' databases/datacenters/servers etc...
if you had one of the "dropped" sites - you may be noticing some Google referrers for some of your keywords - but when you check the links you find nothing
To find out WHERE
these visitors are originating from
use the above link and put in the IP address of the visitor if your Stats/trackers provide them - (if not just - get a free trial of some cut and paste invisible tracker and add them to your sites)
This will show you at what areas of the world the Google Datacenters have begun getting back to their original Algos