Stop right there. Here's how it could go:
Guard: Wait right here, please.
S: Am I under arrest?
Guard: Please wait. What is your name?
S: So I'm free to go?
Guard: Please wait. What is your name?
S: Buh-bye.
A lot of legal questions come down to a game of chicken, unfortunately. Who is willing to bear the most risk?
Maybe if it really was shoplifting, it goes more like this:
Guard: Wait right here, please.
S: Am I under arrest?
Guard: You are being detained until the police arrive. What is your name?
S: Oh, I'm detained? Funny, I don't notice any cuffs. So I think I'll leave now (drops goods & hops on bike).
Guard: (ponders: If I tackle him, what happens?) Calls police.
Now what?