FOP2 shows asterisk status. Do a "queue show" in your asterisk CLI, if you see queue members there, phantom or not, there are *in* Asterisk and you should look at asterisk to determine why/how they got there. FOP2 just display them. If they were added in a dynamic way they can be removed either from the asterisk CLI or from FOP2 too.
If you are using FreePBX, there is a feature code to add agents to a queue, and if you type the wrong numbers, you can add inexsitant members quite easily. If you Dial QUEUENUMBER*SOMERANDOMNUMBER you might end up with a SANDRANDOMNUMBER agent logged into QUEUENUMBER. That is a FreePBX thingy and user error.
Best regards,