It can be after the rear cat and the rear o2 sensor. When the engine is decelerating (slowing to a stop or going downhill, foot off the pedal) air will travel up the exhaust stream towards the engine. Even though it's only for a short time, it's sometimes enough to change the reading of the rear sensor enough to cause the code to set.

 

 this makes sense because the code always seemed to come on when i was slowing down or coasting.

For what it’s worth, I replaced the entire exhaust on my 2000 Legacy Outback (long story). Everything was great for about a year and then the P0420 code popped up. It took me a while to figure out that one main difference between the old and new exhaust was the insulation before the cat, so I wrapped the exhaust tightly up to the front cat and the code has stayed away for over 1,500 mile so far.