Sounds like the opening to an episode of The Twilight Zone: Japan. But this is not sci-fi. And as far as I know the late, great Rod Serling never visited Japan.
But he should have.
It's plausible to think that (a) since her bicycle looks like everyone else's bicycle, and (b) since there are about eleven thousand of them parked in theoretically perfect parallel fashion, she could grow old walking the aisles. That her bike is the one with the furry, marble-sized Hello Kitty thing hanging from the bell on her handlebars doesn't help because every other bicycle has a furry Hello Kitty thing too, even the boys' bicycles (another TZ: Japan episode).
Notice too that she is wearing a high school uniform. This is a solid indicator that her young mind is jam-packed with standardized test questions and answers to the point that she may even forget she has a bicycle. She would be saved, however, by the vague recollection that she has a furry marble-sized Hello Kitty thing hanging somewhere.
No, the reason she might never see her bicycle again is actually very simple (for Japan).
This leaves them as culturally-unspoken fair game for the drunk salarymen who, after an evening of bonding with their co-workers by getting blitzed on Sapporo beer and sake until they can't remember who their co-workers are, realize they have no money left for a taxi and have to take someone's bicycle.
To answer your question: Yes, everyone locks up their bikes. But this is Japan, land of uniformity, every key fits every bike lock - a fact that goes ignored by the honorable Japanese until they are blitzed and penniless.
The nice thing about all this is that no one gets upset about it. "しようがない," they say. Shiyou ga nai, meaning, basically, "Oh, well."
Then they take someone else's bicycle, preserving the unique Japanese sense of uniformity, pursuing the not-at-all-theoretical ideal of everyone riding someone else's bicycle.