Remember one thing Twonn, during clan wars who ever is the highest ranking officer or member of any group or lobby, they have and can and do and will tell the whole group to remove their clan tags so that they can get to the next game quicker ... when other lobbies or groups see CAG as the clan tag for everyone they simply back up ..
Also , if a clan member has CAG in his gamertag already he is allowed to use something else in place of the clan tag .. usually it is the players rank, but some like to be funny and put something funny like "UsuK" or "uMad" ....
What you need to do is approach who ever you saw without his/her or their clan tag and ask them why they did it ..if they don't have one of the valid reasons you then let a squad leader or someone higher know about this ...