- Yes he can be stealthy, mundanes do not flare in the astral, being "lit like a torch" only happens when the character has spells sustained on him or has active foci (or is actively casting a spell or astrally perceiving for mages). Mundades cannot actually be seen by astral beings, they can only see colorful auras that they must recognize with assensing checks, they have to materialize to be able to see them and identify them visually. Living beings do not flare up on the astral plane unless they are awakened critters (such as astral plants) or the previous conditions i mentioned. Astral security is there for the astral, not the physical. The mundanes handle the physical. Otherwise nobody would use physical security anymore.
2 and 3) If the mage is astrally perceiving, he can see the astral security unless said security is trying to be stealthy. The mage, being able to see them, could then roll his stealth (for himself or his team) to try to bypass that astral security. Concealment is a physical power, it does not work in the astral as far as i remember. (i could be wrong on this, i dont have the book on me right nowTip: Never try to use it against a dragon).