I would definitely say that Will should be able to sense the Mara, from what I've been reading about it. Actually, it will probably make him sick.

Hmm, his canon is pretty vague about what exactly he can do. He can speak mind to mind, and he has some ability at mental combat. Theres a scene where he and one of his enemies lash out at each others minds. He might be able to find Five, but I think it would hurt him.

Which could be an interesting way to do it.