Discern location requires that you have seen the target (or possess an item that once belonged to it). What happens if you were wearing a magical or mundane disguise and the caster didn't see the "real you"? Does the answer depend on whether your disguise was a random person, or whether it duplicated someone else who could be a valid target of the spell? Why?
Dream Scan (introduced in Occult Adventures), requires the same targeting as Dream, which states:
At the beginning of the spell, you must name the recipient or identify him or her by some title that leaves no doubt as to identity.
What if the caster only knows you by a false name that no one has? What if you're borrowing someone else's name? In this case, at least, I feel pretty strongly it should either fail or target that real person, which means spies would be better off using cover identities of actual people just in case the opposition tries a remote Dream Scan (or similar divination spell) to probe their memories or find them. Figuring out you got the wrong guy is harder than figuring out the guy you're after is a false ID.
Any thoughts on related issues or spells?
I think in pretty much all cases (except for 'false identity actually belonging to someone else') I would err on the side of "the spell fails". It typically makes for better plot construction.
In addition my own reading (I don't know if this is the consensus interpretation or not) is that the text in the rules about knowing when a creature makes its save ("Likewise, if a creature's saving throw succeeds against a targeted spell, you sense that the spell has failed") doesn't let the caster differentiate between 'spell failed because saving throw made' and 'spell failed for some other reason'. If you go with that interpretation, you leave the players uncertain about why a divination might have failed - false identity, mind blank, made saving throw, is deceased, in a lead-lined room, whatever.
As far as I remember the Vigilante has it clearly stated that divinations targetting his currently inactive identity fail, this way or the other.
As for the seen target thing - it depends if you interpreted the seen target as some other target and are trying to find the PERCEIVED target, or did you specify the SEEN target for the spell. If the disguise was some random person, then it won't help against a "tell me where's that guy I saw back then", but if the question is "tell me where mr. Bumbles is" then the spell will find the real mr. Bumbles, not the person disguised as him..unless of course you've never seen the real mr. Bumbles and that's how you name the counterfeit.
For false names though - yes, I would agree that targetting someone by a false name would fail..or find someone else bearing that name
This is a really hard one. You also have those who are never seen without their masks on (Ex: certain priest types, elite police/soldiers, death squads... etc). If you are looking for one man who wore the exact same uniform and mask as the 9 other men in the group every time you saw him... can you really say you "SAW" him for the purposes of the spell? Could you picture that specific person clearly in your mind? Would you even know him if you saw him without his uniform and mask out on the street?
I don't know if there's a clear answer or not, I will be interested to hear what others have to say about this. I am leaning toward certain disguises being able to thwart this type of magic.
Maybe I shouldn't, but in trying to imagine how exactly this spell works I am comparing it to modern investigation and man-hunting techniques. The easiest way to find a person is to plaster his face all over national media, you would probably find him relatively quickly (assuming a vigilant population). But what if you don't have a face to look for? What if he was disguised as someone else, or wore a mask, or the camera footage is too bad, or what if you simply didn't get close enough to see him clearly? Well now we have to look for other clues... hopefully some DNA. The second requirement for finding a person with Discern Location is to have an item once possessed by that person, something he may have left a psychic impression on, or DNA. Now you can link that psychic impression, or DNA, to a specific identity and positively identify that one specific person.
What this boils down to is two requirements:
1. You must have seen them. When I think about why I am left thinking you must be able to accurately picture that one specific person in your mind in order for the spell to seek them out and present you with a location. So, if you are picturing a person who doesn't exist, or if you have never seen that person without a uniform and mask on, I feel like you have NOT in fact "seen" them for the purposes of focusing this spell.
2. Okay, well if you haven't seen them maybe you can get your hands on something they used to own. Something that they would have touched. Something they would have left a piece of themselves attached to (DNA and/psychic impressions). Now you don't need to know what they look like to start with. The DNA/Psychic Impression will produce an image and focus the spell for you. Not unlike popping DNA into a national database and waiting for a face to pop up.
Now, what can disguise your identity? I would say anything that obscures the details of your appearance. Cloths that conceal any identifying marks on your body and a mask to hide your facial features. An actual disguise, via the Disguise skill and kit. Also magic, from simple Illusions like a Hat of Disguise to more complex changes like Alter Self (That said, we are talking about an 8th level spell and any magic user capable of casting it would likely be able to see through any magical disguises used to conceal your identity). So, believe it or not, it would seem like you would be better off with mundane disguises, or a layered mundane/magical disguise. Which is, again, similar to modern techniques. It's the low-tech people who stay "off the grid" who are the hardest to find.
That's my initial take on it. Like I said, I am interested to know what other people think.
On the other hand, the Master Spy PrC is designed to keep you anonymous and has these as it's 9th and 10th level abilities:
Hidden Mind (Sp): At 9th level, a master spy gains the benefit of a constant mind blank spell at a caster level equal to her character level. The spy can suppress or resume this protection as a standard action. If dispelled, the spy cannot resume the mind blank for 1d4 rounds.
Assumption (Su): The ultimate ability of the master spy is to take over another persona entirely, making it her own. As a full-round action, the spy can touch a helpless creature and shift her aura to that of her target. This confuses divination effects and spells, even ones as powerful as discern location, such that they register the spy as being the creature she has touched. This ability is not proof against the actions of deities or similarly powerful beings. The assumption of an identity lasts until the master spy ends it (a standard action) or she uses the ability on another creature.
That being the case, I could be wrong and maybe powerful counter-magic like Mind Blank or the Assumption ability are the only ways to thwart such Divinations. Then again, the two ideas don't have to be mutually exclusive. Mundane disguises may protect your identity enough to cause a failure of the spell. While powerful magic like Mind Blank is enough to protect you even if you have been seen. And even more powerful magic like the Assumption ability is enough to effectively "become" another person.