Somebody did care enough to create it in the first place, and just not knowing the ownership of the intellectual rights, doesn't mean it's perfectly legal to claim them for yourself. As you say, probably not going to be a problem because you're unlikely to be found out, but not exactly ethical is it?
I used to make software for a business and was robbed blind by people "borrowing" the results of my efforts, and claiming it didn't really matter.
Common "justification" was that everybody does it. No they don't, only thieves do.