Detect magic + Spellcraft = aggravation?

[Steverooo]

One party member has picked up a magic item, unknown to the rest of the group (and he might not even know it is magical). When the party mage uses detect magic to look in an area where the other PC just happens to be standing, does he see the “new” item on the other PC? Does he recognize it as something new (after all, most PCs have a few magic items on them all the time)?

Again, I would give them only the schools (if they concentrate long enough, and succeed in the Spellcraft rolls), and the level and school of the strongest aura. That's all; nothing else. YMMV. They would only notice a change if the new aura was stronger than, or a different type than, the previous "muddle" or auras was. You, of course, can handle it as you please.

For myself, I assume that a distance of one foot is enough to separate mild, Faint auras, two for Moderate, etc. Things closer than that all bleed together.

 

[Steverooo]

A door that will only open to someone speaking the password. What aura, what strength, what spell?

Again, figure out what YOU want! I would say that this is voice-activated, a simple door, with the best lock available, and no keyhole (so no picking possible). The voice-activation (according to the DMG) uses Clairaudience, and I would call the opening Knock. Hence, faint auras of Divination and Transmutation.

If you wanted the door more magically protected, this could be a variant version of Arcane Lock, as someone else already posted, which allows anyone who knows the proper word to open the door, not just the caster. In that case, though, I'd add that variant spell to the caster's spellbook, in case it comes up as treasure!

 

[Steverooo]

A tiled floor that zaps trespassers who don’t walk in a certain pattern. What aura, what strength, what spell?

Hrrrm! WHAT is it "zapping" them with? Shocking Grasp? Burning Hands?

Figure out what, then (from the description) use Alarm as the trigger, and you have two faint auras of Evocation. Fireball and stronger spells might (or might not) raise the aura to Moderate.

 

[helium3]

In my opinion, the magical aura of an active spell is an effect of that spell. Otherwise it's nearly impossible to determine what a spell with a non-visible effect is if you weren't there to see it cast.

I'm inclined to agree with you on this. That being said, why isn't this in the description of the Detect Magic spell? It seems like a pretty important function of the spell. Are we simply supposed to assume that this is how the spell can be used?

 

[Steverooo]

A fresco showing a raging battle, from which a warrior will jump out of when the door to the room closes. What aura, what strength, what spell (and where – the door, the wall, the floor the PCs is standing on?)?

Okay, now you're getting complicated, and stepping outside the RAW, but that's okay, we can handle it! Just go along with me on the newly-invented Object Contingeny spell!

Since the trap triggers when the door CLOSES, I'd use this variant of Contingency (Moderate Evocation) to cast... what's the name of the spell that transmutes anything into anything else? Anyway, it is Strong (9th level) Transformation. Note, here, that I am assuming from your description that the portion of the fresco actually transforms, and BECOMES the warrior. As others have said, Summon Monster would do much the same, and at a lower level, too!

So, mixed auras of Evocation & Transformation, with the strongest aura being Strong Transformation. Where? Just on the door. When it closes, if the Wizzer (Bard, whatever) is still detecting, they will notice that a portion of the fresco becomes magical, but will probably drop the Detect Magic as the warrior steps out...