These days I would be worried if someone got on the plane and then off again...I'd be notifying the flight attendants... too bad you weren't able to find out the rest of the story... did she look like she was looking for someone?
One time I was in a window seat in coach class and a big man, who took up almost 2 seats, was in the aisle..I couldn't get up to go to the bathroom for most of the flight because he fell asleep and I didn't want to attempt to get by him.