Yes, a wrong act is still wrong even if nobody knows about it. An action is wrong if it brings about some kind of harm to oneself or to others. Whether or not a third party is made knowledgeable of that harm is irrelevant, as that harm has still taken place.
Let's say you were in a manslaughter. You hide the body, and people would never have a finger on you. Life is good, and the case becomes cold. Is it still wrong? You took a person's life away and made an emotional impact on their friends and family, may even scared the community of an unknown murderer.
It will always stick to you at the very least, and you will need to say to yourself, what I did was wrong. A human will feel guilt in one self and may even feel shame and turn themselves in for what they done. People may act selfish enough to not say anything, but it will stick to them what they were/are doing is wrong. People around them may not see it, but they are manipulated by that very individual who has done no good.
The human conscience is a very complex yet simple thing to understand. Grief is something that is complex and yet simple at the same time,
Choose to ignore it... It eats you.
Choose to embrace it... It eats you
My name isn't nobody, however, if commit a wrong act I might burst into flames if I don't tell someone...
If it's wrong, it's for a reason. Your actions can affect someone without them knowing it's affecting them. Like, if you are stealing money from your parents and they don't know or realize it, and nobody knows except from you, it IS wrong if according to your moral standards it's wrong.
Youtube - Shelly Kagan vs William lane Craig
In this debate, Kagan explains why doing something wrong is still wrong whether he thinks it's wrong or doesn't think it's wrong, and whether someone knows about the wrong or not. (Without resorting to some vague sense of divine morality).
For instance, is it wrong to kill a hooker who had no friends and family, and the police are not looking for her? Is it wrong if no one knows she ever really existed, let alone was murdered?
If the murderer gets away with it and goes on to live the rest of his life with no one else the wiser and then he dies, and any knowledge of the event itself is lost, is it still wrong?
According to Kagan, morality (in this sense right and wrong) is the terms of an unspoken contract between rationalizing individuals (in this sense, humans basically). Whether he agrees to this unspoken contract or not. Since the morality is what he terms categorical.
The question is not whether it is still wrong if a wrong act goes unnoticed. But hypothetically, would rationalizing people see the act as wrong if they knew about it?
In this sense rational is based on the paradigm of "right" and "wrong". What would a supremely rational group of humans agree is right and wrong taking a plethora of interpersonal factors into consideration. You should have a look at this debate on Youtube (Kagan is a professor of philosophy at Yale, and Craig is a Christian philosopher + evangelist).
He gives a more than sound foundation for objective morals, and also answers your question, is a wrong act wrong if nobody knows about it.
People saying the grief or guilt will eat you up are deluding themselves into an external control nexus. One can choose to conduct oneself in a way where if they screw up they will be "guilty" and will take responsibility for it. However it is only through self-deception that people convince themselves that guilt holds the power rather than being a social practice and a personal choice.
Right and wrong are just human ideas. Morality is the product of ever ongoing social negotiations. The effects of one's actions are real, yet conceiving of these effects as being right or wrong is a subjective evaluation.
Naturally in order to have meaningful lives and not boring ones where we are completely unsure of what to do the fact that it is subjective does not stop us from taking these things seriously.
Still a person who is without delusion will orient their morality more towards their own good and the good of their immediate friends, family, and prospective people one feels will enhance one's life. Not that giving to strangers couldn't be a useful or fulfilling experience and it could and often is but there is nothing objective that makes us all equal, nor is there any objectively correct hierarchy. Social position is the result of a series of exchanges. Since people have various subjective ideas and since to a certain degree there tends to be commonality among these ideas we still seriously discuss these matters.
But let's take off our blindfolds and discuss these matters in full recognition of the fact that these ideas are just human constructs. The only alternative is that you become a slave of others and their word games.
For something to be wrong there has to be a wrong and a right. But wrong and right are just ideas there is no natural wrongs or rights. But if there is a wrong and right depending on the severity, you could drive oneself insanely paranoid, in my opinion. So it could go either way but i just chose no for no reason.