2010- Trouble shoot, Name that problem?

SECOND OPINIONS- seeking second opinions to current dilemma. I think I know what it is, but Im interested in hearing from those Prius owners with experiences akin to the symptoms I'm experiencing now:

- coolant leak from pin hole leak in middle of 2nd CAT underside car, only when car is on idle; engine bay dry; overflow coolant reservoir immediately drains in <30mins
- inverter coolant is not affected and is still full
- while engine on maintenance mode idle, I inspected engine bay for any leaks--- none; except only from 2nd CAT
- however the overflow coolant reservoir is stable and does not leak overnight; except for when engine is on and idle
- I triple checked for re-connection of all hoses and loose or extra bolts
- 1x month back, first recent sign of leakage from coolant overfill reservoir
- I replicated this scenario twice by driving car around block and refilled reservoir; still ends up draining < an hour
- I fixed a lot of things in the engine bay: basic tune up; cleaned EGR systems; 2x coolant exchanges; motor medic engine oil flush; sea foam via air intake, etc
- no creamy froth on oil cap nor oil dip stick
- car has been sitting for months on car pad

 

MJOO,
this evening was a lot cooler, allowing me to get under the car again.
After another good look w fresh eyes, I believe you are correct about an internal crack in the EHR and that the coolant line is emptying into the EHR. The 1st and 2nd CAT seem undisturbed. And for now Im in agreement with you that the individual EHR piece [through age /300,000 miles] just eventually failed. As i look for replacement parts online, it seems like you are correct, that welding is too tight a tolerance; a total front exhaust line replacement may be my only option.
thanks.

 

I mislabeled the EHR as the 2nd/downstream CAT--- hence the pin-hole leak through the EHR. Either way, its the 3rd or last bulb before the bend to the muffler.
thanks.