I'm going to post what I find here so other landlords can handle conversion, trespass of chattel and abandonment of property disputes they have with dead-beat tenants in California. By posting the information, I'm hoping to keep fellow BP landlords out of seriously-hot tenant-friendly legal water.
If you're a residential landlord and the tenant or any other property owners left behind property after vacating the premises, see CA Civil Codes 1980 - 1991 before you even think about moving, disposing or keeping their property. You're obligated to hold it for a period of time. You can't keep it, hold it or sell to make up for unpaid rents, fees or damages. Follow the procedure in the code for "safe harbor"
If you're a commercial landlord and the tenant or any other property owners left behind property after vacating the premises, see CA Civil Code 1993 - 1993.09. Special note, if there are chairs left behind with a sticker on the bottom that says "XYZ Chair company," you're obligated to contact the chair company to investigate if it's their property. On a good note, if you give it to them and the tenant claims the chairs are owned by tenant, you're not held liable for giving it to XYZ Chairs. As the landlord, you give it to whoever said it was theirs first-- and that's that.
With respect to my issue with a landlord that hasn't yet evicted the tenant, is the only key holder to the property (tenant gave up keys) and won't allow me to enter and retrieve my property, there may be grounds for conversion or trespass to chattel. In short, when a landlord is exerting control or dominion over another owner's property, the landlord is interfering with the owner's right to the owner's property.
In a nutshell, trespass to chattel is the "little brother" of conversion. It's for people that want their property back and money lost by not having ownership of it. Conversion is money sought for the entire value of the item and money lost by not having ownership of it. If you won't allow a tenant, lien holder or owner to retrieve their property, they have pretty good grounds for a suit.
I'm making note now that this information I find is with respect to California (tenant friendly state). A lot of these laws seem to be the same or very similar in other states, so check your area. Additionally, taking legal advice from a person you don't know on the internet isn't really wise. At best, I'll be pointing you in the right direction for your reading.
Further Reads / Links:
Is this property abandoned?
Dealing with RESIDENTIAL property abandonment
Dealing with COMMERCIAL property abandonment
Sue for Trespass to Chattel or Conversion?
Conversion Explained
Jury's Instructions: Conversion?
Jury's Instructions: Trespass to Chattel?