Matthew J. Ruff Theft Law Blog on Lawyers.com

The answer to this commonly asked question is no.  The law that regulates the crime of shoplifting in the state of california requires only that an item be taken with the intent to permanently deprive the store owner of his property.  This means that a person can be arrested inside a store as long as the required intent can be shown.  This is commonly done by using circumstantial evidence to prove the intent such as placing the item in a purse or secreting the property on the person.

The legal term for this element is often called "asportation" which in short means the taking or moving of property with intent to steal.  Interestingly, a person could be convicted of petty theft if they do not actually pick up or touch the item themselves, if an individual assists someone else to commit the crime they also can be convicted. 

In summary, a person is not required to leave the premises to commit the crime of petty theft or shoplifting.  Security guards are warned however that arresting someone inside the store will be harder to prove in Court and can possibly result in allegations of unlawful arrest unless serious evidence showing the legal element of asportation is gathered prior to the detention.