Culkin, who became a breakout star playing Kevin McCallister in 1990's Home Alone and its' two sequels, gave the brunette beauty a rose when they met up.
"It was a simple gesture but very touching -- it suggests they must be very much in love," an eyewitness told US Weekly, adding that the actor's appearance was on the upswing as well.
"Macaulay was looking great -- nothing like the disheveled character we've seen in recent years. He's clearly got his act together."
Not so fast: One insider told the magazine The Good Son star, now 33, might also be in the City of Lights to pay a visit to pal Pete Doherty, the English rocker and Kate Moss ex who has a long, documented history of drug-related shenanigans.
"He enjoys being with an extreme character like Pete, and Pete likes him a lot, too," the insider said. "They sit up all night talking about the arts over bottles of wine and putting the world to right. They have become great friends."
Home Alone made more than $476 million worldwide, and spawned two sequels, cementing Culkin as the premiere child star of his time.