Detect Magic and Doppelgangers

[FireLance]

This, and the fact that outsiders and elementals don't register despite more often than not radiating powerful supernatural effects from their bodies, makes me more confident of my position.

Well, the counter-argument is that outsiders and elementals do not have magical auras simply by virtue of being outsiders and elementals. For example, an angel will show up under detect magic not because it is an outsider, but because it has a supernatural protective aura.

 

[frankthedm]

An interesting implication of this ruling is that anywhere that a dragon with 21 or more Hit Dice has used its breath weapon will have a lingering aura for 1d6 days because it is the equivalent of an artifact-level magic item.

The collateral damage a dragon's breath does to the area lasts a bit longer than that.