It's when something comes up and you remember that you read something relevant in the past. In most of those cases it's not about "Ah, this is something I may want to check again at a later time, so let's highlight it". So, there's no list of highlighted passages to search through.

Indeed this is one of those things I miss about paper books. When the above happens I have a terrible time finding that on my Kindle. Most of the time I'm not able to, while with a paper book I didn't have that problem, mostly.