What games system out there have a good ruleset for a covert magic system. For the lack of a better word I use covert in meaning that magic isn't obvious. Not that the magicians would have to hide, but that they aren't slinging fireballs, they are placing curses.
Magic like this could always be hidden as a dumb luck, or accident. It isn't instant and obvious.
Think how in the real world people believed in witches and tribal wizards, voodoo. No one expects them to go out and fly on a broom, but they could give a infertile man a potion and he would suddenly make his wife pregnant (skeptics would say one in a million). A gunman could easily attack a powerful wizard and not fear he has stoneskin and answers with a chain lightning, but he could have a protective charm necklace that would make it seem that the gunman was inaccurate, and later on the gunman would be hit by a lightning, but that's just a coincidence isn't it.
The magic system does not have to be the games default system, but mention what book introduces it then.
Also it's perfectly acceptable that magicians could get powerful and make deal with demons, change the weather and such powerful effects. Just not lightning coming out of his hands.
Some mentions have been that the question is too broad so I'll try to narrow if further. Also there seems to be a bit misunderstanding that I'm talking about the magic being quite or undetectable and while that is a part of what criteria is it's not the heart of it.
I'm not looking for magic system that can make magic harder to see for the sake of it being undetectable, I'm looking for a magic system that models what people in the real world thought magic was like, not what people in a Tolkien fantasy setting thought magic was like. One of those believes was that magic was covert and perhaps it was my mistake to focus on that too much. I'm just not sure what word I could use to describe what I'm after.
In the Salem witch trials I doubt (don't know the subject well enough) that people claimed they saw the woman slinging fireballs. They claimed that a woman begging for money, but was refused muttered a few curse words under her breath and later on all the livestock died.
While this could probably be modeled using D&D magic system, that she used some sort of
remote fireball spell, it's not what I wan't. I want a spell system specifically modeled to represent these kind of spells.
Rituals, spells that take time and need to be prepared, not one you can whip out in the middle of combat. Occult magic would probably best describe it.
We might see this in movies like
Serpent and the Rainbow and
The Ninth Gate and would fit best with a modern setting or a low fantasy setting.