The word is used to refer to taking something without the right to do so. The key part is the lack of a right to do so, not depriving the owner.
e.g. If you take someone's car while they're away on vacation and bring it back before they get home, it is still stealing, even if you topped up the tank and they never noticed.
The reason that it is wrong to take my car, even if I don’t notice, are multiple:
- You are still depriving me of the possibility of using my own car, even if it happened to be the case that I didn’t need it. I could have come home early. I could have told some friend or family to pick up my car so that they could use it. I could have had an appointment that someone would come by to service my car while I was absent. All of this is made impossible if someone takes my car.
- It still causes wear and tear on my car if you use it.
- You could get into an accident, which could hurt you and damage my car.
- I don’t want random strangers to sit their butts in my car.
Not to derail further, but iirc that’s not the case in most western jurisdictions (“intended to return it after using” is actually a defence against theft charges).
Yes, in the legal world there are more specific meanings of words (of which "stealing" usually isn't one) which may vary, but we're just talking about colloquial english here.
e.g. If you take someone's car while they're away on vacation and bring it back before they get home, it is still stealing, even if you topped up the tank and they never noticed.