How would Alucard even know what a Belmont smelled like at the time of Dracula's Curse?
Let's for a moment forget the fact that we're all dealing with supernatural possibilities that are implausable in the real world.
Let's also forget for a moment that these worlds, powers, characters, etc. are not set in stone in terms of abilities, etc.
One could, for the sake of argument, say that a being could have a power like Death's power, where one can see/smell/feel/perceive another's power level (especially if it's over9000).
Since it's never explained, anything could be said about it. It's like saying "Well why couldn't Alucard turn into a Wolf in Castlevania III?" Simply because it wasn't crafted yet.
Similarly, just because it is not crafted, doesn't mean he never had it.
That's the beauty of videogame characters; they're never fully fleshed out. There isn't a manual out there stating all of Alucard's canon abilities because if there were, then no new abilities could ever be added. The ambiguity is the entire reason this thread exists, so throwing monkey wrenches into theories is pretty pointless.
Within the realm of possibility, Alucard could easily have such a power.
And within MY continuity, Alucard met Sonia earlier, which would make things even simpler. You may not like it, and I'm not claiming it to be canon, but it would make the possibility of this ability very easy to conceive.