The way we play the Presence spell is based on all the older posts in the forum, I let him cast it as a full circle, or as a cone extending to the normal radius. This means that if time is not an issue, and an additional unexpected presence is detected, the general location can be known.
But that is just to be able to point the target to the rest of the party, with his own 'm' spells I allow casting as soon as the presence is detected, no need to see the target, as the mind is the real target. That is why I thought that seeing someone that had an unpresence cast on themselves may not be enough to attack with 'm' spells.
Anyway, I understand that it might give too much power to a simple hiding-type spell, and I might rule some modifier to the RR in favor of the target.