Sounds like she was more interested in what she could find in your bag than in poaching your seat.

I have a "return from the lav" experience, but rather than poaching my seat the perpetrator poached my underseat leg space.

The middle of a night transpac flight. Pretty much everyone is asleep, including those around me. (I was in an aisle seat.) Get up to use the lav, and when I come back a few minutes later there is an unknown package filling the leg space under the seat in front of me. Everyone around me is still asleep. Since to my overactive imagination a suddenly-appearing unknown package is, shall we say, somewhat suspicious, (and I don't like having my leg room poached), I immediately find an FA, who comes over and wakes and asks the sleepy passengers next to and in front of me if the package belongs to them. All answer in the negative. The FA starts to take the package away when the perpetrator pipes up (from three rows away mind you) that he is the owner. The FA returns it to him. I notice that his underseat leg room is open. My overactive imagination imagines kicking both him and the package off the flight somewhere over the North Pacific.