Is there a word for something that does NOT belong to someone?
Solution 1:
for something not owned by someone you say "not his." Example:
The bicycle is not Joe's.
For something that doesn't belong to anyone, I would use "ownerless." Example:
The stray dog is ownerless
Solution 2:
For intellectual property, you can say: It is in the public domain (not covered by copyright and hence belongs to no one)