Can anyone who doesn’t live in my home affect my bandwidth usage?

That depends. If you use a router and have not secured it, someone nearby with a computer or Wi-Fi enabled device could access your home network, use your high-speed Internet, and increase your usage. Common encryption protocols, such as Wireless Encryption Protocol (WEP) or Wi-Fi Protected Access (WPA), can provide additional security. You should contact your router manufacturer for more information specific to activating your router encryption. You can also check your router manufacturer's support website for help in securing their Wi-Fi gateway.