User blog comment:FairyRaven/Fairy Tail Chapter 392: You Must Not Forget, Review and Commentary/@comment-24800928-20140711144951/@comment-24066070-20140711153231

Or maybe it becomes more difficult to distinguish the more smells are present.

Ivan (when posing as Alexei) was in the capital, together with thousands or even tens of thousands. Furthermore, Makarov was also present whose smell must be similar to Laxus/Ivan and could "confuse" Natsu and other DS.

Gildarts and Cana may have different smells since they aren't the same gender, as you already said. Besides, Gildarts was always away (and sleeping with various women with different perfumes) and the moment Cana and Gildarts would be together was when they are in the Guild Hall where there are many smells mixed together.

In Silver's case, there weren't many people/Demons in close proximity and Natsu would realize there is no way for Gray's smell to suddenly show up in Tartarus HQ, thus making it easier to trace it to Silver.