Guys, how close in proximity do you need to be to a girl you're attracted to in a large public place in order for you to recognize that you know her?

Like say you're grocery shopping. Is it likely that you don't realize that she (a girl you know from somewhere else) walks past you because you're busy concentrating on the task at hand (and therefore she winds up being the one that acknowledges and says hello to you)?

Guys, how close in proximity do you need to be to a girl you're attracted to in a large public place in order for you to recognize that you know her?
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