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The speculation has been there for months, but when Jonas, 19, showed up to Swift's concert on Sunday night, many onlookers thought it was "real obvious" he was there to see his gal pal.
"He watched only her performance, then walked backstage to the meet and greet area," one concertgoer told PEOPLE. "But when he saw people start to recognize him, he hid. It was real obvious he didn't want people to know he was there to see Taylor."
Swift, 18, told the magazine in July the two were just "friends."