For those of you who doesn't know, you can return your octopus cards in the
train information booth at the airport and get your 50$ deposit back. This
however needs to be done before passport control as there is no similar
booth inside.
I didn't test this for myself though, as I was running short for my flight.
If anyone knows otherwise he/she can update here.
Sent from my Android based tablet. This on screen keyboard is challenging
and may cause typos.