I thought Tabitha was the group of seven's eye and Cynthia was their healer. I think Cynthia's detection range is simply better than Clare's, so she was doing the sensing.

Rubel is interesting - he seems to be quite strong based on the panel we saw a while ago of him wielding a claymore with one hand, but Clare's group couldn't detect him which leads me to believe that he does not have any yoma energy. I also suspect he may have his own purposes that are separate from the organization but is unable to do anything alone.