Hmm no, it seems you pretty much hit the mark on that one, Shien. The only thing I really feel like adding would be that a sniffer's sense of magical energy - and the information one can derive from observing this - is actually one innately possessed by all living beings, though to vastly different degrees. Sniffers have extraordinarily keen senses in this regard, but others - especially mages, and necromancers even more so than other magi - have the same abilities. It's as Shien said: even regular people can pick up on unusually powerful presences, which is also one likely explanation as to why people seem to automatically put distance between themselves and the demonic duo.

So basically, all of the information that can be garnered by a sniffer could potentially be gained by others as well; a masterful necromancer would be able to sense an amount of information almost as great as that of a sniffer, a master mage quite a bit less than that, a regular person very little... and tarken are, of course, all the way down at the bottom of that particular ranking. It would take almost deity-level power for a tarke to be able to sense it.