"So random," Fowler said. "The guy tried to say it was a gift. I don't know why you would give a gift of a golf ball with an earplug attached to it."
The fan was escorted from the property by police.
"It was one of those things," McIlroy said. "It was a golf ball with an earplug stuck on it, so it was sort of strange."
Fowler, the 2012 winner, finished with a 71, leaving him 6 shots behind Steve Wheatcroft and Andrew Loupe, who each shot 7-under 65 in rain and steady wind to share the first-round lead. McIlroy, the only two-time winner of the tournament, shot a 73 after a bogey-plagued start to his title defense. He won by 7 shots last year, finishing at 21 under.
The talk of Thursday's round came after McIlroy hit his tee shot on the green on the 250-yard, par-3 sixth hole. When Fowler stepped up to the tee, he saw something fly by his left side, startling him for a moment.
He didn't know what it was until it stopped rolling.
McIlroy wound up with a birdie, while Fowler and the other member of their threesome, Hideki Matsuyama, carded 3s.
McIlroy has some work to do to get into contention if he hopes to become the first three-time winner.
He called his first competitive round since the Masters a "tale of two nines."
The world's No. 3-ranked player shot a 4-over 40 while starting on the back side, then got things headed in the right direction with four birdies on the front.
"I was sort of still in range mode on the front nine," McIlroy said. "I didn't really have my scoring head on."
McIlroy, who turned 27 on Wednesday, wasn't pleased with how he played in the soggy conditions but said the strong finish gave him something to build on for Friday.
"I need to go out and shoot a good round, something in the 60s, and get myself back into the tournament going into the weekend," said McIlroy, who has spent the past three weeks working to fix his swing.
Rory McIlroy entering a busy stretch
ESPN senior golf writer Bob Harig discusses the busy schedule facing Rory McIlroy over the next several months.