The culprits were none other than "X Factor" girl group Fifth Harmony, who left a message for their British mentor.
"Happy 87th Birthday Simon!! Love, Fifth Harmony," read a banner the group had hung over his driveway.
"Verrrrrrrrrry funnnnnny @FifthHarmony," the 54-year-old media mogul tweeted Monday.
"LOVE YA SIMON!!! Happy birthday @SimonCowell," the girls wrote back.
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