The Fixer isn't a necromancer or has any real practice in hiding the strength of his soul, so he generally doesn't even try... and even if he had tried, a Sniffer would be able to sense it anyways. Their magical sensory powers are unrivalled, after all. That being said, the Fixer will give off an aura that Morgan will probably recognize as being very powerful, but not monstrously so in the sense that it is still within the range one could expect from a mortal, rather than being on immortal-level. He's certainly stronger than even most experienced mages, but not altogether fearsomely so.
How far can a Sniffer's abilities go... eh, see, the thing is that being a Sniffer means to have taken one's control over one's own soul away, removing all conscious and instinctive influence on the flow of one's energy and instead enforcing the rule of the Sniffer tattoos. They can't actually control their abilities (which is why Sniffers get bombarded with readings on everyone around them constantly) or expand beyond what they were initially. They do possess a certain degree of passive absorption of ambient energy similar to liches due to their natural flow being turned inwards, and since energy can't actually leave their souls again their souls grow in strength the longer they remain Sniffers, but they can't actually use that energy for anything. The passive energy absorption is also so slow that its effect on spells cast near it would be quite neglectible, and unless someone was intentionally pouring raw, unmanifested magical energy into the Sniffer they wouldn't be able to harness more than a very small amount of it. Sorry, but Sniffers don't work that way; their abilities are almost exclusively sensory.