You would think a detail like that would be explained in the story. That’s just strange.
The group of people who rent scooters contains an inordinately high number of people who find it more convenient to dump them in a nearby lake than to return them and pay for the time over the rental contract. Scoots are also easy to steal and joyride and what do you do with a scoot you have stolen when you are done with it? If it is at the bottom of a lake it will not be found likely for a long time, if ever, and there is little chance of recovering fingerprints et al from it.