My suggestion is that when a cleric casts sanctuary, all nearby players should have to roll to perceive the character or not, regardless of their disposition towards the character (friendly/neutral/hostile). I know when you cast sanctuary right now, friendly targets are still able to perceive you.
This strikes me as an issue in pvp. For example during a large NCE pvp encounter today, one of the adversaries stayed blue while attacking, and basically cheesed through the sanctuary. While this is probably against the rules, the reality is that this kind of thing is probably always going to happen. So it might be better to adjust the spell in the way described above.