Another P0171 story

The victim: 2002 Gen I Prius, 287k miles, half in FL, half in the Northeast. MPG about 39.

Started getting intermittent CEL with DTC P0171. Cleaned MAF & throttle body no joy. Replaced MAF, cleaned throttle body again, no joy. In fact, frequency of DTC increased.

So thinking to start looking for vacuum leaks (next on the fault list), I looked under the car, and it was noisier than I liked when running. But no apparent leak in exhaust system. So I decided to patch the exhaust leaks that I suspected were behind the shield that Toyota welds to the bend in the exhaust just past the cat converter and prior to the HCAC. Removed the shield (cut close to welds; about half an hour with a Dremel and a couple of cutting wheels), and revealed much corrosion and cracks in the pipe... hence the exhaust noise.

Patched the holes and cracks with muffler putty and covered the pipe section with high temperature exhaust tape, and I'm surprised, right now the DTC is NOT coming back after 30 miles of driving.

So a question
if there is enough leakage just past the downstream O2 sensor, is that a possible cause for P0171?

<edit> so, after a few more miles, the code is back... it just took longer than previously (sometimes it would occur in 6 miles). Did not need to be in a WOT (Wide-Open Throttle) condition to set the code.

BUT: after patching the holes in the exhaust, the car is a LOT quieter when running on ICE...

Question #2: if folks have fixed P0171 with a vacuum leak, where did they find the leak?