What IS it with yuri manga and throwing in the shot of the "mysterious" woman in trendy clothes walking through a terminal or a train station to show that she's coming from somewhere far away? And having her say something ambiguous that obviously hints at a history with one of the leading ladies? There's clichés and then there's this!
oh no a cliché the horror.
Although I do like this particular cliché because it usually ends up with the leads progressing their relationship one way or another. there will be bad drama but it will leave development in its wake.