Bono made a surprise appearance on Sunday night at Bemelmans Bar at the Carlyle Hotel in New York, where he joined singer Jim Caruso and pianist Billy Stritch onstage for a handful of standards, the Huffington Post reports.

"Bono walked in. It was impossible not to recognize the guy – the glasses alone gave him away!” Caruso recalled to the Post. "They were seated in the corner, where they rocked, bopped, swayed and snapped. He kept giving me the thumbs-up when he'd like a tune and singing along quietly."

Stritch and Caruso were performing tunes from the Great American Songbook, a set of classic songs that the U2 frontman knows well. Before Bono left, he headed right for the stage and joined in harmonies to "I've Got You Under My Skin," a track he recorded with Frank Sinatra in 1993.

"The room was stopped cold," Caruso said. "Just another miraculous Bemelmans moment! After the last note was crooned, Bono flew out the door, waving to everyone. Billy looked at me and said dryly, 'Well... that happened!'"

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