2 incidents:
Astrally Projecting Fomorai Shaman decides to cast "Phantasm" to make the illusion that he and all his awesome spells have no astral presence in the area, effectively he's "invisible."

the enemy spellcaster casts "Trid Phantasm" to have the illusion that he is no longer here (cannot be seen, touched, heard etc...), and then a phantasm to cloud the astral forms.

in both instances if an observer shifts his perception to the astral plane will the mage be visible?

Hm... well i play it that everything is truthfully visible on the astral plane.

Any invisibility spells and such are easily discovered.

SR20A page 208 in the illusion spells segment it states that, while mana-based illusion do work on the astral... you can just assense them and done.

Only thing going into astral stealth is masking-metamagic. (Or just mindcontrolling the enemy mage to just ignore you *g*).

Yes, you're visible in astral.

yes but in the above examples there isn't an illusion to assense, the illusion is that the target is "gone" in the second example they could assense the area due to "hey he vanished where'd he go?" but if you don't take the action to assense then you don't see anything right.

so it would seem the query is whether or not there is sufficient evidence to warrant a further inspection of the specific area?

Well, they would see some kind of huge astral entity... And you are hiding just in it. Not really stealthy...

A spell itself is always "there". You don't need to assense it to see it. You just need to assense it to be sure and get additional information.

"Though mana-based illusions can be created on the astral plane, their magical auras give them away as illusions to anyone who makes a successful Assensing Test."

the illusion is not "given away" until you make the test. if the illusion is that there is no mage, then you don't "see through" the illusion until you assense, you don't see the area affected, you'd see the area as if the target was not there. but an assensing test to anything within the area should reveal the information required to disperse the illusion.

thanks for the reply

But they have obvious auras. The sentence is referring to non-invisible illusions, but you can't hide the aura anyway; that is, you see the illusion of a person, assense it, and know that it's an illusion.

The short answer: there is no invisibility on the astral.

Where to begin with the long answer....

First and formost, the way I read the (trid)phantasm spell is that you can create an illusion of X rather than being able to conceal Y (the realm of mask/invis spells)

If you are allowed to use the phantasm this way on the physical plane (or just use invis) it will do nothing on the astral so not only is the casters aura visible as normal it will also be surrounded by the active spell's aura as well.

You can cast mana based illusions on the astral but they are easily noticed as such with an assensing test. Also, the listed invis/mask spells only affect physical appearances, not astral ones. If your GM allows you to 'hide' using a phantasm spell (or you custom design an astral invisibility spell) you could conceivably hide your aura that way.....but unfortunately the spell's aura is still plainly visible and you can tell exactly what the spell is with enough hits on an assensing test. Masking/extended masking metamagic would allow you to conceal the spell aura in your own but masking alone does not hide an aura, only changes its appearance. I would find it a bit of a stretch to be able to conceal a spell's aura in the caster's when the spell effect is to make that aura invisible

Hope that helps

Edit: I have to type faster....there were no responses when I started this post

As a side note on P 208, SR4a under illusion spells

"Illusion spells cannot fool assensing to disguise or create auras"

Trid Phantasms best use is to block LOS, for my money anyway. I've never really thought about using one in the astral, but I don't play a lot of awakened. An astral onlooker knows immediately that it's a spell, but he still has to resist it to target beings on the other side. That's my read anyhow.

Yeah, it's *possible* for one aura to be hidden behind a larger/brighter aura in the astral. I feel like Aarakin's quote means that you can't intentionally use illusions to do this on purpose, though.

It's a little beside the point, maybe, but I personally wouldn't allow Trid Phantasm to create illusions of absence. Mainly because the description of the spell doesn't make it clear that it's capable of that, and because if you were to build a spell capable of that with the Street Magic rules, the DV would be two higher.

Actually what they want is in Digital Grimoire. See the Illusion Spells false impression and manascape.

first off I am not saying you are wrong, as each person's interpretation is just as valid as another.
if it said "create image" or "create object" or similar then I would agree. however it says "illusion" and an illusion is The condition of being deceived by a false perception or belief

AFAIK, False Impression is for changing one positive aspect of an aura into another. A human into an ork, not a human into nothingness.

a false perception is just that, you perceive something incorrectly.

Right.

Whoop

True, they technically would want to use trid phantasm and then manascape . After that the entire team loaded to bear + back up spirits could look like a shrubbery.

Hey Bob?
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Definately...