Are the “voters” in question party affiliated?
If they registered fraudulently they would most likely register as a member of the opposite party since you can vote for whomever you want in the general election.
In CA the registration form has a space for a physical address (required) and a mailing address (optional.)
If you’re homeless, you can register with an approximate location and use general delivery as your mailing address.
If you’re properly registered at a real residence and then become homeless, you can continue to use that address until you register again.