What you're thinking about is the "Merchant Privilege Rule". It is under this "rule" that a private person can detain another private person without effecting an arrest. Security Guards should be VERY aware of this rule. Also, companies that employ security guards and/or their own loss prevention people will generally indemnify those employees. The employees won't be paying their own legal fees unless the company determines that the actions of the employee were clearly outside the scope of their employment.
PC 490.5
(f) (1) A merchant may detain a person for a reasonable time for
the purpose of conducting an investigation in a reasonable manner
whenever the merchant has probable cause to believe the person to be
detained is attempting to unlawfully take or has unlawfully taken
merchandise from the merchant's premises.
(2) In making the detention a merchant, theater owner, or a person
employed by a library facility may use a reasonable amount of
nondeadly force necessary to protect himself or herself and to
prevent escape of the person detained or the loss of tangible or
intangible property.
(4) A merchant, theater owner, a person employed by a library
facility, or an agent thereof, having probable cause to believe the
person detained was attempting to unlawfully take or has taken any
item from the premises, or was attempting to operate a video
recording device within the premises of a motion picture theater
without the authority of the owner of the theater, may request the
person detained to voluntarily surrender the item or recording.
Should the person detained refuse to surrender the recording or item
of which there is probable cause to believe has been recorded on or
unlawfully taken from the premises, or attempted to be recorded or
unlawfully taken from the premises, a limited and reasonable search
may be conducted by those authorized to make the detention in order
to recover the item. Only packages, shopping bags, handbags or other
property in the immediate possession of the person detained, but not
including any clothing worn by the person, may be searched pursuant
to this subdivision. Upon surrender or discovery of the item, the
person detained may also be requested, but may not be required, to
provide adequate proof of his or her true identity.
(f) (1) A merchant may detain a person for a reasonable time for
the purpose of conducting an investigation in a reasonable manner
whenever the merchant has probable cause to believe the person to be
detained is attempting to unlawfully take or has unlawfully taken
merchandise from the merchant's premises.
(2) In making the detention a merchant, theater owner, or a person
employed by a library facility may use a reasonable amount of
nondeadly force necessary to protect himself or herself and to
prevent escape of the person detained or the loss of tangible or
intangible property.
(4) A merchant, theater owner, a person employed by a library
facility, or an agent thereof, having probable cause to believe the
person detained was attempting to unlawfully take or has taken any
item from the premises, or was attempting to operate a video
recording device within the premises of a motion picture theater
without the authority of the owner of the theater, may request the
person detained to voluntarily surrender the item or recording.
Should the person detained refuse to surrender the recording or item
of which there is probable cause to believe has been recorded on or
unlawfully taken from the premises, or attempted to be recorded or
unlawfully taken from the premises, a limited and reasonable search
may be conducted by those authorized to make the detention in order
to recover the item. Only packages, shopping bags, handbags or other
property in the immediate possession of the person detained, but not
including any clothing worn by the person, may be searched pursuant
to this subdivision. Upon surrender or discovery of the item, the
person detained may also be requested, but may not be required, to
provide adequate proof of his or her true identity.

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