ANSWER:

This is money earned unlawfully. 

If there is a policy or a way you return this to the company, you should do this. But if there isn’t any official way or it would put you in haram way,  you must give it to poor or needy Muslims to cleanse your wealth from it.

I used to work at a company ‘X’. I used to take 15 minutes to go get food and return back to work, however I would not register this as a lunch break. Essentially, I was getting paid when I should not have been paid during these 15 minutes Is this considered stealing? If so, how do I repent? Should I return any money? I was getting paid hourly at this job