Yes, that was my thought as well, you lock onto the auras, and if they leave the area it still affects them, if others come in, it doesn't. Was just wondering if there's maybe a rule I didn't see that states differently. Doesn't seem to be the case.
For indirect spells it's obvious. If you walk into the Wall of Fire you get burned, if you step out, you stop burning. Though I'm not even sure there are any indirect sustained area spells.
Physical spells are a bit more iffy I think, but since they can affect objects as well, I usually see them being anchored at a point in space, and radiating out from there, so leaving takes you out of the effect, entering gets you affected.
Hmm, I was always under the impression, once a spell is cast, you can sustain it no matter the distance, you just needed the LOS or Touch for the initial casting.
Also it's Magic x 100 meters for spirit's remote tasks (p302), and I guess it would make sense that sustaining stops there too.