For a long time SEOs suspected that Google was indexing URLs that are found with in content, but not linked to. To be honest, I never believed it myself, and then not long ago I was proven wrong [via].
“Sometimes it happens that we pick up a whole URL like that. Sometimes someone will try to shorter a URL with just a ‘…’ in between and we try to crawl that URL so we get it wrong. But our goal here isn’t necessarily to pass any pagerank, which we don’t do with those kinds of links. But rather discover new URLs that we haven’t seen before. And if we see someone write about a URL that we haven’t seen before we will pick that up and try to index that for search.”
"Sometimes [..] Sometimes" Googlers speak for the masses, which means "Sometimes" is likely to mean "not often at all". But of course that won't stop the chatter on the interwebs!