My understanding is that in bypass, any bypass, a unit is always in one of the hexes that form the hexspine and NOT in the other hex. An AFV (or infantry or any unit) in narrow street bypass is in one of the hexes the narrow street divides and NOT in the other hex. That's the short answer to your question. Well, actually, Klas gave the short answer. Mine is the medium answer.
The long answer is that it wouldn't be ASL without some quirks and caveats and B31.11 explains the exceptions around smoke -- for example -- if a narrow street separates hexes A and B (which are same level hexes) then smoke in either hex A or B effects movement costs on both sides of the road ... But that leaves me wondering about smoke hindrance on the narrow street.
Suppose:
An AFV is in bypass along the narrow street using the hex A side. Hex B is filled with smoke. Seeing as Los to targets in bypass is traced to the CAFP... would a line of sight traced to the AFV's CAFP that does not touch the smoke in Hex B anywhere but at the CAFP (if it touches there) be subject to a smoke hindrance penalty?