I learned that Logos will not search highlights even when highlights are filtered in the Notes. When searching for a word or topic, Logos only searches the "notes" within (or attached to) the highlight. It cannot search for any string inside the highlight itself. Which is strange given that the book that I highlighted is searchable.
It seems an obvious need for a search function in "Notes" that includes "highlights" where "highlights" can be filtered to be able to search for a string in the highlights.
This was confirmed by Logos tech support.