My mage player asked me, if he could just astrally track the mage who sent a spirit who was following him.
I said sure, there is a paragraph in the rules for this and then I had to think a lot, how to describe something that would fit these rules.

Here is, what I came up with and then some questions about stuff I couldn't decide on (I will call the mage who sent the spirit "the target"):

It seems that during tracking the tracker does not gain any information about the target (no initial direction or approximate distance or something like this) until the tracker has finished the astral tracking. At least nothing of the kind is mentioned in any way.
This means that the link cannot be a direct link (like a rubber band between spirit and target). If it were the mage could just begin tracking, see in which direction the link points and then stop tracking and just fly in this direction.
How fast does the mage move in astral space while tracking? Since the tracking takes the same time if the target is in the neighbour's house or in Casablanca the mage can obviously move very fast while tracking. Normally one would think that the mage would move very, very slowly if the target is in the neighbour's house, because he takes several hours to get there. But that can't be, because then the mage would only have to begin tracking and then stop after an hour to see how much distance he has traveled to get a rough estimate on how far away the target is ("I guess he's still in the city. I travelled only a few hundred meters during the first hour of astral tracking." or "I guess he's left the continent. I was in near Chicago after the first hour.")
Because of this I think one can imagine the astral link like a very, very twisted line in astral space that is several hundred kilometers long even when the target is only 50 meters away. Let a shoelace fall on a map of America or even the world and then only adjust, that the ends are at the spirit's location and the target's location. That would be they way I see the astral link.
Until the end of the astral tracking the mage has no idea where the target is and he cannot draw conclusions from the speed with which he tracks, because of these enormous detours.
Why should the astral link be like this? I don't know, maybe it's a deflected to various locations the target thought of since the spirit has left him. Main reason is game balance. If it would give you any hint about direction and distance in just one hour it would be too useful.

Additionally: I can't imagine that the whole astral space is criss-crossed by thousands of astral links. So I would rather think that they are only visible at their two endpoints and you can only "see" them when you are tracking one and then you can only see the one track that you are following.

This leads to my first question: if the begin of the track is something mobile (and intelligent) like a spirit, how near the entity must the mage be to begin astral tracking? In the scenario from the beginning: the mage has noticed that he is being followed by a spirit und wants to know who sent it. The mage tells his chummers to protect his body and astrally projects and tries to hide from the spirit by dodging through a wall into a warehouse. He amazingly succeeds when he tries to hide behind a box (infiltration is not his best skill, obviously) and sees that the spirit followed him inside and is searching for him.

1. Can the mage now begin with astral tracking (just by having LOS to the spirit) or would you say the mage has to stand very near the spirit ( at the end of the astral link) to begin tracking (and with that it will be nearly impossible not be noticed by the spirit).
What I would like to know, is how you handle a mobile origin (of the tracking process) like this.

Another question this time concerning a mobile target:

2. The mage has begun tracking. At this moment the target is not protected by any ward and the tracking has a threshhold of 5. The mage begins to collect hits. Two hours later the mage has collected 4 hits but his target returns home and is now protected by a rating 5 ward. The mage continues to collect hits and cannot find him, when he has reached a threshhold of 5, because he now need 10 hits, correct?
And if the targets steps out of his ward during the time, when the mage already had more than 5 hits but not enough to find him behind the ward, I will find him instantly?

3. After the initial two hours of tracking, the sun sets. The spirit vanishes and from then on the mage needs one threshhold more per hour because of the "active" line in the tracking table. Or does the link just vanish with the spirit?

How do you handle these situations and how do you describe the astral link and astral tracking?

you're onto some very good questions here. Nobody really knows.
There was a Magic source book planned a while ago, but they dropped it with the leave of their best writers. It would have covered everything there is to know about the astral space. I was so looking forward to this. But nowadays they prefer printing more guns, more tanks and more guns, because they need less thinking about. As I said, the creative brains are gone, and we will never get answer to your questions.
The only thing we can do is find a consensus, about what the experienced players believe.

1. The answer is successful Assenssing, I believe
2. I agree
3. no idea

Funnily, astral link is not on the "Assensing Table" on page 191, so we don't have a given threshhold for this test.
But I agree that an initial assensing with at least one hit is necessary to see the begin of the astral track.

After some thinking I have decided that for balance reasons the tracker should be at the spirit 's side to "enter" the track.

I image the begin of the astral link like an umbilical (the silver cord of earlier editions IIRC) that fades out of sight after a few feet. As the tracking mage you have to "touch" this "visible" part at the beginning of the astral link and then you can "see" the rest of it and can begin tracking.

In case of the the astral link being attached to an astrally projecting mage (to find his body) or spirit (to find his summoner) you would have to succeed with an astral infiltration roll against his assensing to do this.

About question 3:
I will probably rule like this about astral links that are deactivated: if the mage has already "entered" the astral link meaning he has already begun tracking, he can follow the rest (this getting more difficult per hour).

It's a pity that they never described how this tracking process might look like, but I think I have now found a way for me that fits the rule, looks nice and is not major contradictions in itself.

IIRC astral tracking takes a threshold of 5, with a time of 1 hour (modified by the rating of any ward that the target is behind)

I've always thought of it like a scent trail, and it fades with time. That being said, yeah the threshold would go up if the target entered a ward. If they then left the ward, not sure. But just for game play's sake I would not decrease the threshold under the theory that leaving the ward is like going down stream to shake a dog's scent. Now if the target went in and out of wards I'd only use the highest rated ward.

Cool rule Warlord.