Legal Question in Real Estate Law in California

If your brother still has a key to get into the place then he still is a renter and need not give notice to the landlord if he is still paying rent. If he is not paying rent and claims he is no longer a tenant then what legal right does he have to walk into the unit? He would be a trespasser.

He should return the keys, contact the owner to see what is a good time to remove the rest of his and your furniture, and notify the people still living there so they are not afraid he may take some of their stuff.

"Staying at" does not clarify whether he was a tenant, subtenant or a guest, nor does "just recently" provide a lot of help, either. Nevertheless, I can say that as a general rule, he should notify the person or persons with legal occupancy rights, which would perhaps be the tenant of whom he was a subtenant, or the owner if there is no tenant. Notifying other occupants would be helpful, but is not legally required. Whether he does this informally or formally depends entirely on his relationship with the remaining occupants. If he is jolly good pals with them, a phone call an hour or so beforehand should be sufficient, unless he needs a forklift and a moving van to get all his and your things. Ih he got kicked out, or owes money, etc., I'd suggest a written notice followed by a phone call a day or two before, and be prepared to pay the rent or whatever is owed. Use common sense and ordinary courtesy.