There's a difference between borrowing something and stealing it. I think the situation you described isn't exactly morally wrong, although there are certainly better ways of doing so. But really stealing something is wrong whether someone notices it or not. If I were to break into a rich man's house and make off with some plates, he may never notice, but that doesn't mean it isn't unethical.
Stealing something is wrong because it deprives another of their property, but it's also dangerous because it definitely gives rise to greed and disrespect in the mind. Although "right" and "wrong" are definitely vague terms, I would say that any moral system should have as its highest goal reducing harm and generating positive mental states such as respect and patience and non-greed and all those other things that an act of stealing destroys. In a strictly secular sense, it's wrong, but even in a purely moral sense, it's equally wrong because it leads to destructive mindstates.
Stealing something is wrong because it deprives another of their property, but it's also dangerous because it definitely gives rise to greed and disrespect in the mind. Although "right" and "wrong" are definitely vague terms, I would say that any moral system should have as its highest goal reducing harm and generating positive mental states such as respect and patience and non-greed and all those other things that an act of stealing destroys. In a strictly secular sense, it's wrong, but even in a purely moral sense, it's equally wrong because it leads to destructive mindstates.