I can use it because it has never been published.
I can use it because I found it on the Internet and there was no copyright notice.
I can use it because I’m not making any money off of it.
I can use it because I am only using a small portion of it.
I can use it because I changed a lot of it and made it my own.
All of these thing can make you liable for copyright infringement. Now, whether you get away with it and don’t get sued or kicked off your IP is another matter.