Fuel smell question

Picked up an 05 with 180k for my daughter at $1000 in the PNW, which is a great deal for up here, carfax shows hybrid battery replacement at 135k. Suspension is new and honestly it runs GREAT.

The young guy who sold it to me (sold it due to new job teaching English overseas) how its hard to fill - he kept trying to top it off. IIRC there is a fume filter or something up there that is probably full of gas now.

But when it's driving it seems fine fume/vapor wise). At any standstill (start or light signal) the vehicle very quickly smells like a mix of fuel and poorly catylated exhaust.. it's also throwing an O2 sensor code. So at this mileage maybe the catalytic convertor is old and that's the sensor issue. But the vapors are way too strong. And there is that gas smell too. Could that be from overfilling?

I'm guessing it's not fuel injectors leaking or we'd pick up gas smell from open vents. The smell seems to come up from the backend through the frame or floorplan or trunk somewhere. Not sure how he kept his baby in the backseat with that level of fume.

We're pulling the interior for a deep clean (she didn't pay for the car so she has to put sweat equity into it for appreciations sake) so it's fine to yank whatever inside or under the body, but what I wanted to know is if you guys had any clues or hints where I should start looking?

It may be multiple issues, but I don't know this car at all; wife had an '08 I tinkered on in '09 but that, and her, are long gone and so are the memories. I ride motorcycles 12 months a year and fuel-line, or filter issues, or exhaust issues are something I've never even thought about.