The security settings were marked as "Unlisted" (Only people with the link can see this video).
In 24 hours, the video gathered 48 views and 3 comments. One comment started off with:
Landed on this video searching for something else "…<topic>…" related, and wanted to throw in my two-cents. Reason being, is I have done my share of <topic>, and I share your pain brother. Just hoping to shed some 3rd-person perspective on your situation.
Here's where I'm confused…
The link was shared with only three people, including myself
I didn't share it beyond those two people.
The second person was obligated (and I really do mean obligated*) to not share it.
Before I go leaping to what seems to be the obvious conclusion here (that person 3 shared it), what else would you suspect?
Discuss/Explain?
I'm particularly interested in explanations that also tie up the "I was searching for other <topic> things" aspect neatly.
GO!
Unlisted doesn't mean invited people only. It's just a way to limit viewing through an unprotected, invite means.
I think there's still another way to literally have it private and only selected emails can view it, even if they try sharing it with others.
So unlisted just means not inserted into the search for easy finding, but, if it's legit, and it could be, a person could still find it by accident. Unlisted doesn't mean private.
IK on FB if you only share with certain people or just "friends" it can show in their postings anyway and is viewable by anyone they are linked to… So it really is seen, and shared, by many more.
This seems to be the most definitive answer – YouTube itself:
Unlisted videos
Making a video unlisted means that only people who have the link to the video can view it. To share an unlisted video, just share the link with the people who you’d like to have access to it, and they’ll then be able to see it. Unlike private videos, the people you share the video with do not need to have a Google account to see the video, and they can share with more people simply by forwarding the link to them.
Unlisted videos won’t appear in any of YouTube's public spaces, like your channel page or search results.
So, although the video is not Private, neither is it searchable, and Occam's Razor says that Person #3 shared the link, and that's where the 40-odd views and the comments came from.