In Star Wars: The Force Awakens, Rey often leaves her speeder completely unattended. Why is this? Thievery appears to be very common on Jakku.
It also features a fingerprint scanner to ensure that only she is able to use it.
The factbook "Rey's Survival Guide" also offers the following info. In short, even if someone managed to steal her speeder, they'd probably crash it within seconds. On top of that, Unkar has told the local low-lives to leave her ride alone.