1) Costco is a private shopping club and they can revoke your right to shop there at any time. Read your membership agreement.
2) Actually, the constitutional argument doesn't apply here. Unreasonable search and seizure is written to reign in the government, not private citizens. If a store employee so much as touches a customer that person can file an assault complaint and ask that charges be brought. If one is "detained" there are also charges that can be files since security or the vest-wearing employees are not LEOs and don't have the right to arrest someone.
3) At least that is my understanding, if I'm wrong on any of this please correct me.
1) You are correct.
2) You are partially correct here. Your forst 1/2 is right, but store security does have the right to make a citizen's arrest (if specific legal criteria are met) and use reasonable force, if necessary to effect their arrest.
[REVIEW THIS INFO] California Penal Code Section 834 states: "An arrest is taking a person into custody, in a case and in
the manner authorized by law. An arrest may be made by a peace
officer or by a private person."
Section 837 states:
A private person may arrest another:
1. For a public offense committed or attempted in his presence.
2. When the person arrested has committed a felony, although not in his presence.
3. When a felony has been in fact committed, and he has reasonable cause for believing the person arrested to have committed it.
Section 839 states:
Any person making an arrest may orally summon as many persons as he deems necessary to aid him therein.
Section 841 states:
The person making the arrest must inform the person to be arrested of the intention to arrest him, of the cause of the arrest, and the authority to make it, except when the person making the arrest has reasonable cause to believe that the person to be arrested is actually engaged in the commission of or an attempt to commit an offense, or the person to be arrested is pursued immediately after its commission, or after an escape.
The person making the arrest must, on request of the person he is arresting, inform the latter of the offense for which he is being arrested.
3) You know we will!
And how violently would you refuse?
How about if they circle around you?
How about if one grabs (or "places his hands") on your arm?
I agree w/you BTW. Since I paid for it, it's now MY STUFF. My line is, "you think I'm stealing, call a cop and arrest me- and we'll find out for sure". The only goofy deal is, it is their shopping cart its all in, so they could grab the cart...
My response would be, "Are you placing me under a legal citizen's arrest?" If the answer is "no," I walk. If the answer is "yes," the sext question would be, "for what charge? (see section 841, above)" Then, I would demand they have an officer respond without delay and that they turn me over to the officer immmediately.. see the (a) section below.
Section 847 states:
(a) A private person who has arrested another for the commission of a public offense must,
without unnecessary delay, take the person arrested before a magistrate, or
deliver him or her to a peace officer.
(b) There shall be no civil liability on the part of, and no cause of action shall arise against, any peace officer or federal criminal investigator or law enforcement officer described in subdivision (a) or (d) of Section 830.8, acting within the scope of his or her authority, for false arrest or false imprisonment arising out of any arrest under any of the following circumstances:
(1) The arrest was lawful, or the peace officer, at the time of the arrest, had reasonable cause to believe the arrest was lawful.
(2) The arrest was made pursuant to a charge made, upon reasonable cause, of the commission of a felony by the person to be arrested.
(3) The arrest was made pursuant to the requirements of Section 142, 837, 838, or 839.
If they claim they are placing me under citizen's arrest and disclose the code or crime they are arresting me for, and agree to turn me over to the police without unnecessary delay, then there would be no need to put their hands upon me. I would make it clear to them that I was submitting to their citizen's arrest and I would not accompany them to their office, I would demand they have the police respond to our location.
Hands-on force, any forced/coerced movements and taking me to an office or detention area would add battery, kidnapping and false imprisonment to the false arrest suit.
eight 35 oz. bags of Dove's wonderful small premium chocolate squares. A little re-arranging of the items in the seat bag, which had been left unmolested behind the wheelchair, some careful stuffing of the chocolate bags into the bag, and I was good to go. 
