I think it is either a symbol to say that Xur is at the tower, or it's meant to display where he is.
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Necrobump
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It's just to let you know xur is in the tower, also there are purple flowers above the the emblem
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I just use an app called Where is Xur on my IPhone to locate him. It even tells what he is selling. I'm sure it's on the google play store as well.
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