There is a hidden reserve slot for the admins and reserved slot purchasers. They can only access the extra slot if they use the console to connect to the server but it lets them take the 31st slot then kicks a player out so they can then move to the 30th slot. That slot should only be filled for a matter of seconds just to let an admin on the server.
Also could just be a glitch of someone trying to connect to the server. A lot of times before I had a reserve slot and waited forever to join i'd be able to see people trying to connect but soon fail. :]