How long ago was this? If very recently, maybe their documents haven't cleared. If a long time ago, maybe the records weren't complete.
And how close is this neighbor? I believe Zillow still doesn't cover every county in Michigan. I lived near the county line, so my neighborhood was fully covered, but one we can see from our back door had no info.
And why don't you want this to be made public, anyway? Are you worried people will find out you are a shrewd bargainer? I have had some potential buyers try to reference my purchase price when negotiating (last sold just last year, prior to moving to Ireland) but that's really irrelevant vs. current market price.