JMich: I think they are posts from deleted and/or disabled accounts, so you only encounter them if you go grave digging.
Ah, that would make sense, and would explain why I've never seen any - I very seldom go digging. Not sure why anyone would want to hide those, though. And how in the world did you find those posts in the first place??
As noted by Vitek in the interim, and in light of
this post - upon examining the dates of the other posts in its thread - it's clear that the choice to post anonymously was an option sometime during the beta phase, and if deleted / disabled accounts are one source of those posts, that is the other.