I blame the hangover.
Arriving at the Jetstar check-in counter in Melbourne I reach into the slot in my bag where I am absolutely certain I've left my passport. Absolutely certain. It's not there. Problem.
We'd left the hotel early. Retracing my steps, I recall seeing my passport sitting on a table in the hotel room as I was heading out the door the night before.
"Probably safest to pop it in my bag, out of sight," I thought. And that's what I did.
Absolutely, certainly putting my passport in the side pocket of my carry-on shoulder bag.