Improved Invisibility, invisibility, How do they work? Options

[FriendoftheDork]

Hey guys, after reading the invisibility spells I am somewhat confused. I'm used to invisibility in D&D etc. where when you cast the spell no one can see you, thus they cannot target you without finding you with spells, hearing or scent (and pinpointing by hearing is exceedingly difficult).

In SR, all it says is that you become invisible to anyone who does not beat your threshold (hits), and that anyone attacking you recieve the -6 blind fire penalty.

But how can they attack someone invisible at all, unless that character is obvious in some other way (firing guns, attacking you in melee etc.)?

The situation that I have invisioned is that an NPC the party has a contract on is a shaman. His magic rating is only 2, so he's not THAT dangerous, but he can cast the spell and since they have no counterspelling the likelyhood of the PCs failing a Willpower (2) check is immense (the troll has WP 1!). If the party is unlucky and they all fail, they could risk the shaman stunbolting them every turn (force 4 at overcast with only 1 physcal drain!). Force 4 would require a threshold 2 Perception+Inituition check, which means that the PCs are likely of detecting spellcasting, but is this enough to pinpoint the invisible character (who really might be hundred yards away)?

My gut feeling tells me, that in order not to unbalance this spell (and risk the PCs taking it later), the invisible shaman should be autodetected when firing guns or in melee, and if they detect spellcasting they should also be able to pinpoint him enough to attack him. What do you think?