First thing I would do is look at the client list (or DHCP list) to see if there're any devices that have connected you don't recognize.
2nd, change your wifi passphrase and reconnect only your known devices.
If your router has detailed logging as a feature, enable it so you can see which client is the most chatty going forward, and check next month to see if traffic died down. If not, check the logs to see which client was responsible.
First thing I would do is look at the client list (or DHCP list) to see if there're any devices that have connected you don't recognize.
2nd, change your wifi passphrase and reconnect only your known devices.
If your router has detailed logging as a feature, enable it so you can see which client is the most chatty going forward, and check next month to see if traffic died down. If not, check the logs to see which client was responsible.