We did not travel on a R/T ticket.

The incoming ticket and the outgoing ticket were booked independently and flew on different airlines.

We flew in on CX which the check in agent at HKG did ask me the print out of the ETA.

We flew out on QF. The agent wanted the US passports (naturally because we flew to LAX). The security check staff wanted to see both passports and matched the details of both. But after that was done, he motioned us to put away the HKSAR passports. It was this little extra step that disrupted our travel routines since 2012 - that the US passports were only used when we ARRIVED US soil, for the re-entry of US Else it is always the HKSAR passports that see us thru immigration around the world. The little "lounge" of QF F check in and the extra security scrutiny of flights to US upset our routine and caused my totally forgetting we should use HKSAR to exit Australia.

Both tickets were issued completely independently without any linkage. In other words, 2 PNRs though both were from same issuer - Alaska airlines issued it as their mileage awards. Upon entering Australia the immigration officer did a lot of checking on her computer with my HKSAR passport, and asked me to take off my eyeglasses but never asked me my return ticket.

I was surprised at exit the eGate opened with the US passport when I later thought about it - as it should not have any information of this passport associated with the current ETA. Back in May of 2009 was the last time the US passport being used for the ETA and subsequent entry/exit.