[jerryme]

I want a ghost to defend a place against magic and come to the physical plane to engage the mage as soon as he casts a spell.

How do I keep the mage from ruining the surprise by looking astrally through walls? Mana-barriers seem to be see-through and are also a tell-tale sign...

I would like to give the players a logical explanation how the ghost could watch over the place without being seen.

[Lumen]

Any spirit with the Concealment power could stay in the astral and any astral perception tests are made at -Force. If It's a powerful enough spirit or the mage's Assensing isn't that high it could reduce the dice pool to nigh impossible chances of spotting the ghost.

[Michael Chandra]

Free Spirits also have an aura masking ability that you could borrow, where they can disguise their aura.

[Chrona]

Also Ghosts can pretty much do whatever they want (GM wise), being one of the weird "everywhere but not understood" wild spirit types.

[Medicineman]

How do I keep the mage from ruining the surprise by looking astrally through walls?

Astral Perception is NOT looking through Walls. You need Astral Projection for that !

And a slight Background Count (1 or 2) distracts the Mage and raises the Force of the Ghost

He who dances with Ghosts
Medicineman

[jerryme]

Thanks for the pointers, guys 

Actually, after a recent discussion, we decided to try and play an alternate universe where metahumans don't have magic or resonance. We rebuilt the respective chars as a mundane version as close as we could and they suddenly had an GP- and Karma-overflow, of course. So far, we kind of like it. Definitely less things to worry about for both the players and the GM.

[RHat]

...  You'll need to be careful with some stuff.  Per RAW, it is completely impossible to permanently deal with a free spirit or free sprite absent Banishing or Decompiling.