The Neighbors star was spotted, overnight bag in hand, apparently leaving Michelle's Los Angeles home on Sunday, the pair having enjoyed a night out with pals on Saturday at Chateau Marmont.
Zac was also carrying a skateboard as he eased his way out through a side entrance and made his way to his car, his morning hair tucked under a baseball hat.
An eyewitness exclusively tells E! News that Zac and Michelle were hanging out in the back garden of the West Hollywood hot spot with a group that included Robert Pattinson.
It's unclear if this was the first time the hot-'n'-heavy-in-Italy duo had seen each other since Michelle returned from New York, where she spent her 36th birthday last week with other friends.
A source told E! News that the Fast and Furious 7 star partied with pals at Mister H in the Mondrian SoHo and that she seemed "very single" during her time in the Big Apple.
Michelle shared an Instagram video of herself chowing down, no holds barred, on a chocolate cake with a candle stuck in it, captioning the funny clip, "Thanks for the Hotel BDay Chocolate fix I'm skipping lunch today lol ;)"
Our insider characterized what's been happening between Michelle and Zac as a "summer fling."
"They aren't dating per se, but I'm sure once they see each other again they might be all over each other again," the source added.
Meanwhile, we're told that Zac spent Thursday through Sunday at Michelle's place, and that while some friends were seen coming and going and Michelle left for work during the week, he stayed put.
"Michelle is super-cool and laid-back and Zac has always been attracted to her because of that," a source said earlier this month after the first pics of the two locking lips in Italy surfaced. "She's one of those people that everyone likes, she just gets on with all kinds of people and doesn't take the fame thing too seriously."
"Zac likes the fact that she is laid-back and he can relate to her," the source continued. "He is attracted to her great attitude. They understand one another."
—Reporting by Sara Kitnick, Baker Machado and Michelle Falls