The typical problem is the resident manager who has a key to your apartment, and snoops around while you're away at work, going through your things, perhaps even taking your property. It's a great feeling of power for a little person to be able to so grossly invade someone's privacy, like that, and the practice is alarmingly common. There are landlords who give a key freely to workmen to come into your unit, unsupervised, to so what they please, perhaps even catching you in embarrassing moments. There are landlords who freely give a key to realtors to walk into your house any time they please, placing a "lockbox" on your doorknob for any of them to use to walk right in, or set up an "open house" in your home, and ask you to get lost for a few days. There are Peeping Tom landlords, and those who freely come into your back yard to pick fruit and see what you're up to. It matters not that they own title to the property. These are all violations of the right to possession, not of title. 

     They think, "Who cares about your privacy? You're only a tenant."  Try complaining about it, and you get either an indignant denial or a threat of eviction, not an apology. They are wrong, but act like you're way out of line for wanting privacy.  This is because few tenants sue over it. Many are too intimidated to sue their landlord for walking into his own building, or believe they have no rights.

    You can sue the landlord and whoever else comes in with their permission. Sue the landlord and whoever for up to $10,000 in small claims court for trespassing, breach of contract, invasion of privacy, and breach of quiet enjoyment; if you are two or more tenants, each can separately sue them for up to $10,000, and a joint action is not required.  Entry of the rental unit by the landlord or his agents may be a criminal trespass, as well. Penal Code 602.5 and 602.8 [see below] define the trespass as entry and refusal to leave without the consent of the owner "or the person in lawful possession" appears to suggest that whoever is in lawful possession is the one whose consent is required, and where the landlord has rented the possession, the landlord has the money but not the right to possession. There is no definitive ruling in California law, whether tenants can charge their landlord with criminal, as opposed to civil, trespass. Maybe your case will make the law books.

    Under Civil Code 1954 [see below], the landlord may enter your unit without your permission ONLY: 
(1) in an emergency, like a fire or broken pipe, or 
(2) upon reasonable advance notice, and then ONLY:
         (A)  to inspect, repair, or show the apartment, 
         (B) during normal business hours [presumably Mon.-Fri. 8AM-6PM]
         (C) 24 hours is presumed to be sufficient notice
         (D) You do not have to be home when they come, but the landlord is liable for anything stolen or broken.
         (D) The notice must identify a date and reasonable time range [like an hour] within which the entry will occur
         (E) The notice MUST be written [not oral or e-mail], except if a WRITTEN notice that realtors will be showing the property is given, for the next 120 days only an oral telephonic 24 hour notice is required [business hour limit still applies]
         (G) The right of entry can't be "abused", so that an open house, lock box, extended repair, daily entry, or excessive range of entry time are probably all "abuses" which you have the legal right to prevent.

    The nature of the reasonableness of the notice seems to be to give you time to pick up the place, secure pets, arrange to be there, or otherwise make ready for the visit. The law is not clear, here, and the above is the best interpretation of what the law probably is.

Practical Solutions

    These entries can be restricted by practical means, since there is no legal authority (pro or con) to do so:
(1) Write a letter to the owner, carbon copy to the local police, about the manager's burglary. 
(2) Change the locks, or add chain lock to the door [example], so that the manager's entry is restricted. You should plan on moving, if the owner does not restrict the manager's entry, but at least your privacy will be protected meanwhile. 
(3) Sue the manager and landlord for their trespass and invasion of privacy, in small claims court [maximum is $10,000]. Maybe they'll think twice next time. Each time, it's a different lawsuit. Each person so affected can separately sue.
(4) Circulate a flier to all of your neighbors about what happened. Not only can they take precautions, but many may move, punishing the landlord with lots of vacancies. Many may choose to join you and sue the manager and landlord, too.
(5) For the realtors showing your house, call the broker who has listed the house and tell them that no one is to enter without reasonable advance written notice, like 24 hours, and that they should immediately remove the lock box, or you will do it when you change the doorknobs. Tell them that any realtor or prospective buyer will be sued, and the police will be called to charge them with criminal trespass [see above], should they dare violate this. You can also tell each prospective buyer about all of the problems with the house, and the neighborhood, so that the brokers will not want to show the property, at all. In the alternative, you can work out a cooperative arrangement with the broker to ensure that your privacy is respected. They'll get the point.

    The law now requires a landlord to provide dead bolt locks on the doors and adequate locks on the windows of a residential rental unit. Civil Code Section 1941.3. Failure to do so violates the "warranty of habitability", permitting the tenant to move out, repair and deduct, or withhold rent as remedies. You can use that law to put them on, deduct the cost from your rent [See Repairs], and have the additional barrier to their illegal entry.