If you see a 'returned powerbank' when click on an individual slot in a station but you don't have a record of the rental starting it is most likely just a powerbank that was recorded by the station with no actual rental starting.

If a station has some dust, as an example, that comes between the contact on the powerbank and the contact in the slot on the station, the powerbank might not immediatley be registered in the station. Typically just giving the powerbank a tap or a jiggle will get the station to record the powerbank.