Standing front-row at her Oct. 11 MDNA show in Los Angeles, the single actor, 52, couldn't take his eyes off the iconic star, to whom he was married from 1985 to 1989.
"Their chemistry was off the charts," a fellow concert-goer raved in the Oct. 29 issue of Us Weekly. "Madonna seemed to be performing entirely for Sean. And he was saying things like 'Amazing!' and 'She's so hot!'"
The pair, whose constant fighting throughout their brief marriage earned them the nickname "the Poison Penns," met on the set of the music video for the singer's hit "Material Girl" in 1985 and wed just months later. In 1986, the pop star released her third album, True Blue, which was inspired by and dedicated to her new husband, "the coolest guy in the universe."
The relationship was far from perfect, though, marred by hot-blooded outbursts and arguments. In 1989, they called it quits and divorced. Penn went on to marry actress Robin Wright, and Madonna wed director Guy Ritchie -- but the former loves never could let go of each other completely. In January 2010, they broke bread over a three-hour dinner at Adour Alain Ducasse, sparking rumors of a reconciliation.
"They've remained close friends," a pal told Us. "But they are fire and fire together romantically. She can only be with someone she can control."
That's certainly not Penn. And in any case, Madonna already has a boyfriend: Brahim Zaibat, 26, whom she has been dating for the last two years.
"Madonna's friends wish she'd get back with Sean," the source said. "He will always be the love of her life."