[81MC]

An exhaust leak can make the o2 sensor read leaner than it actually is, which wouldn’t typically throw a code. But you’d probably hear something.

What’s more probably, and harder to diagnose, is just an o2 sensor going bad. Assuming it’s mechanically sound otherwise, o2 sensors can start to go bad without throwing a code. Typically this will have that negative fuel economy impact you’re seeing. But you’d have to do some serious comparisons and see what’s more economical - throwing sensors in it vs. the realized fuel economy loss.