I'm trying to come up with a plot where my character is possessed by a supernatural being via magic. The problem is, our cabal covers all of the ten Arcana, which means that, no matter what I chose, it would be immediately evident to one of them that something was wrong whenever they had their Active Mage Sight on, and I'd really like to keep it a secret. I thought Veiling spells would cover this, but apparently it only works for Peripheral Mage Sight. I've talked to the GM, but he seems inclined to follow the game mechanics. Any thoughts?
Yes, but not the way you think, and not forever.
Mages are the greatest detectives in the universe, at least as far as Chronicles of Darkness is concerned. It is by design that very little can be kept a secret from Mages for too long, and for that very reason, really successful Mages are masters of misdirection: make your opponents think they have the truth, so that they stop looking, or just never give them a reason to look in the first place. I like to call this technique, layering.
The simplest way to hide supernatural stuff would either be Prime 2 Veiling spells, or another Veiling spell from an Arcana aligned to whatever you want to hide. Some Veiling spells can be improved with a Reach to make them report false information, and we can assume with Creative Thaumaturgy that this can apply to any spell you need. So, make the Veiling spell report that you have something more benign instead, such as a Fate blessing or something like that. Then, apply a second layer of misdirection. You can either Veil your Veiling spell or put another spell on yourself that calls more attention to it.
To finish it off, don't make anyone think you are hiding something, or just make it appear like you are hiding something else. Mages tend to be very busy with their own mysteries, and it shouldn't be like they have a lot of spare time to scrutinize everything they run into with great detail.
Of course, if the mystery you are hiding is really really interesting, you are doomed to be discovered. There are infinite ways they can catch you and figure out what you are hiding, from seeing your past to reading your mind, just to name a few of the less esoteric ones.
If you want your character to conceal his magical influence to Mage Sight, you can use the following spells.
First, you have "Suppress Aura", a Death 2 Veiling spell (page 129-130 of the Mage core book). It specifically says that the subject appears as a Sleeper to Mage Sight and that magical attempts to see through the disguise provoke a Clash of Wills.
Second, you have "Supernal Veil", a Prime 2 Veiling spell (page 168). It can be used to hide the supernatural being possessing your character. It specifically says that passive abilities such as Peripheral Mage Sight automatically fails to detect it and that active abilities such as Active Mage Sight provoke a Clash of Wills. If your character is remotely controlled by a spell, Supernal Veil can be used to hide that spell instead.