Where's my underwear?

I discovered a small woman's camisole in my luggage when I unpacked it this morning. It has me very puzzled. I found it inside the small gym bag that I had placed inside the large duffle bag. Why this makes it very odd is the only time I did not have the gym bag in my possession was the flight from Sydney to Perth. I had stuffed the gym bag into the duffle before I checked my bags at Sydney Domestic for the flight to Perth. This means that someone went through my stuff after I went through customs and put it in there.

Nothing else in the luggage seemed to be disturbed or missing. I wonder if some poor woman somewhere in Australia has a pair of my underwear in her luggage?