If you cut and paste an asset to another branch in the asset tree the change is not noticed by the search system until you make a change to the asset. Rerunning search.pl also seems to fix it up.

You can reproduce this by setting up a search on a sub branch - copying an asset into that branch, checking that the search finds it, then cut and paste the asset out of the branch and see that the search still finds it. If you then edit the content or change the URL the search no longer erroneously finds it.