Let's say I have an app stored as a closed source code (e.g on github) for which I have a software license in place (Apache 2.0), is this enough to prevent/prosecute an employee or just a random outsider who somehow managed to get the code, copy it and thereby "proclaiming" it as his own, and in what specific circumstances is it possible (him publishing the app, giving it to a third party etc.)?
You can’t prevent “theft” of your source code, but you can ensure that you can sue the “thief” for copyright infringement and receive damages, or force them to delete all copies of your source code.
Now a license doesn’t do what you seem to think it does. Copyright law, without any license, allows people to do very little: It allows them to have and use one copy of the code that you gave them and nothing else. A license allows people to do things that would be illegal without the license. So you need to check the apache license to see what exactly it allows others to do. For example, the GPL license allows people who received the software legally from you with the license to give it to others under some conditions.
But I don’t think there is any license that allows people to just take your copy from you.
If you think someone took your software without your permission, you can ask a lawyer for help. You can as a first step send them a letter that you believe they have your software without permission, ask them to destroy all copies, and notify you that they have done this.
Penalty for copying your software in the USA is either between 30,000 and 150,000 if you can’t proof the amount of damages, or the amount of damages you can prove. Of course this requires that you win a lawsuit which isn’t cheap, and that they have money to pay you.