What is the ruling on using an open wireless internet network that serves the room for 15 minutes, without the owner's knowledge to cover the costs?

Some networks are public and may be used without permission, while others are private and may not be used except with the owner's permission, even if they are not password-protected. If you do not know whether a network is public or private, and you cannot reach its owner, then seek Allah's forgiveness for what you have done and regret it. Nothing else is required of you.