a note that might never be found is not as effective as an anonymous note mailed to LE if you really thought you had a useful tip.
I wonder if the note leaver thought LE would just throw the note in the trash, take no notice or it wouldn't get to the person it needed to at police HQ.
At least leaving a note at Sandra's house, it's guaranteed that one of Sandra's relations will see it....and they left it there, because they wanted the family to know about this potentially dangerous suspect, not necessarily LE.