I deleted the mystery nodes, and so far they haven't come back... so far. In the past, they've come back, but this seems to be a longer period of time. I did review all my rules and alerts, and disabled a few that I'm not concerned about or are not applicable. Maybe I incidentally fixed the problem?
Unfortunately I was not very scientific at all about my efforts, so have no good info to report :-(. Like the post from last year, this one might end just as "the problem went away." My apologies to any future Google searchers!
Thanks again to all for suggestions-- all avenues very instructional about the workings of LEM.