Robert_Charlton - 6:59 pm on May 9, 2012 (gmt 0)
Digging further, I started looking at sites I linked out to from a directory site of mine. I found some that I removed months ago. Some are places out of business. A few have been renamed and have new websites.
I continue to observe that whenever Google surfaces anything, it's very likely that they are testing against it in some way.
Just thinking out loud here... could it be that not everyone has been as good as you have been about removing dead links, and that many links are broken or aren't redirected properly? Perhaps in some cases, redirected sites are spam. Perhaps Google is developing procedures to help purge things more quickly if they should be purged, perhaps to locate the new resources otherwise... or to reassess both backlinks and their own indexing procedures.
In an AI environment, the resurfaced site references might be a kind of a control group... perhaps just a way of obtaining additional user data as triangulation points.
It's like a time capsule with some old crawl got dumped back into the mix.
What you're describing reminds me of something else we discussed here about a year ago, but in some ways never resolved. The phenomenon had the same museum-like quality to it....
Domain name replaced in SERPS with alias domain name
Instances of redirected domain names displayed in the serps thinned out after a while, but from time to time I still run into them.
I'm only taking a stab at this. I don't know what Google might be testing or how, but perhaps someone can build on these thoughts.