It wasn't the wrong bus. Well, that depends on what you mean by the wrong bus. Even know, she can only narrow it down to two possibilities.
One:
She had changed her mind at the last moment, boarding the bus for the convalescent home because she didn't dare to see any of them, not the Sisters or Dr. Turner. She didn't want to deal with Sister Monica Jone's outbursts or Trixie's pert remarks or Chummy's sympathic murmurings that she'd been through this as well, because Chummy hadn't, she wasn't even a novice at the time. And she especially didn't want to face him. She didn't want to look at him and wonder if he really meant what he'd written in that letter, or if he only said it because of her situation.
But as the wheels turned, she couldn't ignore the fact that they were taking her away from both her choices, the old family and the (oh, please God, may it be true) love of a godly, worthy man. Raising her voice—imagining Sister Evangelina dealing with an unruly patient—she asked to be let off at the side of the road.
Two:
The old skirt and blouse felt strange after so many years in a habit. She was used to taking small, quick steps in sensible shoes, to the silent hem brushing the dusty floor of Nonnatus House and the dusty Poplar tenements. Black and white, often stained with crusty brown dried blood.
It didn't matter that the outfit was decades out of date; she didn't care about the stares of the other passengers on the bus. She was used to being identified at a glance, classified and filed away as a nun and a midwife, a good woman and reliable in a crisis, a pair of hands and a calming voice.
"Sister—" the woman corrected herself. "That is, your bus is here."
"Thank you," Shelagh gripped her suitcase tightly, stepping onto the bus without looking at the driver.