Upon looking at my video stats today (I typically check a few times a week), I've noticed that one of my unlisted videos are surprisingly getting pageviews. According to the definition of 'unlisted video', it means only people who have the link can see the video. I looked into where the views were coming from.
(This video, if you were wondering, is one of my perfect game recordings on Solitaire Blitz, doing a full 67 card run without drawing any cards from the deck. I switched its setting to unlisted after it was panned for playing too slowly. It's three seconds short from a perfect time bonus, however, which makes it insignificant. You can see it here.)
Apparently, the only pageviews linking to this video came from not direct links or external websites, but instead from Google Search. I did the keyword search myself, and apparently Google has no problem linking to an unlisted video.
I cannot rule out the definition of 'unlisted video' is wrong because it explicitly states it can't be found publicly on Youtube. Although some unlisted videos can be found online using search engines using some very specific keywords, I am more surprised that one of my unlisted videos can be found with conventional keywords. (the search term was 'solitaire blitz perfect game' in this case)
If this is the case, I wonder what privacy issues can arise when people who have belief that their videos are private are having their unlisted videos being viewed by other people.
Summary: If you want a video to be completely unlisted, make sure you have it unlisted when you upload it before it goes live. Otherwise that video, whether it's unlisted or not, is not guaranteed to be search-proof.