You can check for the list of unknown devices connected to the modem under connected devices option on your modem GUI or web manger portal. Otherwise you need to contact your service provider for more information on this and to stop if some one hacking your devices.

Launch a web browser, such as Internet Explorer, Firefox, or Safari.  Enter http://192.168.0.1 into the address box, and press the Enter key.  The https:// Redirect page will appear.  
The Username field has admin populated, enter the custom password in the Password field.
Click the Login button.  The Gateway > Summary page will appear.
On the left side menu select Connected Devices option and on the page you can see the devices that are connected to your home network.