I think it's more likely to be Observation haki. I'm always keeping in mind that Haki really hasn't been elaborated on all that greatly, and Luffy has only managed to learn the basics from what Rayleigh said.
It's possible that Haki develops generically and/or uniquely in individuals, which makes a lot of sense for it being a supportive type of ability rather than a main ability. Usopp perceiving the aura of individuals is perfect for his sniping role.
We still need some explanation on Sanji the Human Torch's abilities and Zoro's Asura.