It’s… Just just the smell of baked bread and yeast. Anyone that makes their own bread knows what’s going on with the smells in subway and can easily replicate it. I worked at one when I was younger there’s absolutely nothing nefarious or secret about it lmao. I personally think it’s the yeast more than anything. It’s a smell that used to be really common but is much less so these days so it sticks out. A lot of subways have the bread proofing/rising right up by the front too