I know it is an assumption but I would have thought that if boarding was taking place and no one was in the seat that the items may have been inadvertantly left behind. This is based on my experience that passengers are not allowed use of the toilets so where would the missing passenger be? I will admit that 90% of my flying experience is international and I have seen passengers asking and being refused the use of the toilets, but domestically this could be different. I would have thought domestic the priority was to turn the aircraft around quickly, especially with QF OTP trailling VA, hence no time to look for missing passenger and cross reference off manifest, all of which at our desks seem prefectly reasonable expectations that crew and fellow passenger would do, and get items off board for previous passenger.
I am glad that items were returned to the OP and that we all take this experience as a learning, just like when I left my wallet on board a plane after an international flight to SYD. Went to baggage service to see if cleaning crew found anything they radioed back to say no. Later on I received a call from a daughter of a cleaner who said they found my wallet, sans money, and I met up with her for the retreival.:-| Wallet stays on me now and some notes in the destination currency in a my pants pocket.