He didn't sense anybody's presence, if I recall correctly. If you meant he sensed Bonez, he didn't: he could hear the breath of his inert steel, but the living presence of the guy himself.
Observation doesn't work with objects as they lack intent, hence why Enel couldn't predict Luffy's random attack or why the environment disappears but living auras remain when a character is "seeing" through it (Fujitora, Usopp). Zoro even states he isn't sensing presence but something different he can only label as the breath of things. Only future sight gives you a detailed image of what is going to happen, but that's totally different and doesn't fit either as it doesn't involve "sensing" nor "feeling" presences but seeing directly.
The breath of things clearly wasn't conceived as haki, just retconned as such in such a cheap and unexplained way that any attempt at making any sense of it will fall short and, therefore, using it to assure Zoro awakened observation in the first part of the journey is pointless and dishonest.
An obviously retconned power that makes no sense as any form of haki. And a wasted opportunity to build something interesting with swordsmanship, too.