They don’t «smell» it like we think of the word «smell». It’s more like: pieces (chemoattractants) fall of the bacteria and attaches to receptor on phagocyte (because high consentration of a substance = higher chance of said substance to «collide» with the receptor on the phagocyte).

And when a «piece» binds to the receptor it causes an intracellular signal in said phagocyte -> phagocyte shoots out lamellipodium (which basically is a grappler that binds to the ecm in the direction of the receptor binding, and contracts pulling the phagocyte in that direction) -> repeat until catching your target.

I don’t know if that made any sense? Or if it was an answer to your question? The evolution is rather uncertain i believe.

Edit: Fagocyte -> phagocyte