Though you seemed to have handled it admirably, I'd advise something like this:
(Person comes over to ask if you are LEO)
You: "What right do you have to ask me that?"
Him: "Corporate policy, (yada-yada)
You: "You might want to speak to Brandon Palmer. He is the Regional Manager of Applebee's here in Central Washington. I'll be letting him know you are CONTINUING to harass lawful customers, and will be filing a formal complaint if you persist. Come back when you've spoken to Brandon, here's his number".
IOW do NOT answer his questions. He has NO right to ask you your occupation.
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IMO, and I'm just being an advocate, since I never have to answer these questions, you are teaching people to invade your privacy here by answering you are not LEO. The BEST method is to ask a question, this puts the ball in their court and they have to respond to you. If you are engaged in answering their questions, they can put you in unfavorable light. After asking a question, you should move off if you're sure of policy. One needs to teach them that they get nowhere doing this. They can't insist you start obeying their unlawful instructions, ISTM, if you deflect LEO questions not to suggest you are LEO but that it's none of their business what you do. If they persist, I'd leave my groceries and walk out or if a restaurant, ask for a refund and leave. Always run your recorders, it's your only defense if the manager tries to dissemble. Hanging around just gives people time to pile on, which depending on your overall plan, may be a good or bad thing.
$.02
I presume you had your recorder running? No? Tch.