Obviously it does depend on the spell. Some spells are more visible then others. But straight buffs, are visibly detectable at casting based upon force, but not after that, and as far as I know they can hang out astral. Now of course the spirits are visible in the astral and of course sustained spell are visible in astral, so yea from the Astral point of view this is very not subtle.
As to the mod question I think there is one in 4th, a bad guy mage packing pistol with alternating dmso/viper poison rounds and something else who also had some spirits. But I could totally be wrong about that, it's been long time sense i read any of those mods, and they weren't always rules accurate.