Unless you set your server to Private = True, then anyone can join since it's just listed in the server list.
I expect it's just another player who just happened to join your server, either on purpose or by accident.
If you do set Private to True, make sure you add the players you want to your whitelist. Spelling including upper and case matters. When I added "jhadred" to my whitelist, I couldn't join until I figured out my name comes across as "Jhadred".