The duo was performing at the Bemelmans Bar at the Carlyle Hotel in New York, when the Irish star joined them for a rendition of I've Got You Under My Skin - a track he once recorded with the late Frank Sinatra.
Singer Caruso tells the Huffington Post, "Bono walked in. It was impossible not to recognise the guy - the glasses alone gave him away!
"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 (was) singing along quietly.
"The room was stopped cold (when Bono hit the stage). (It was) just another miraculous Bemelmans moment! After the last note was crooned, Bono flew out the door, waving to everyone."