AKRON, Ohio — His day of practice done at Firestone Country Club, Brooks Koepka headed to his courtesy car in the players’ parking lot, which was cut off from the public by temporary fencing covered with banners.

Before disappearing behind the fence, Koepka rebuffed an autograph seeker who had a pin flag to sign, explaining that he was not feeling well. He drove 3 miles to the hotel where many golfers were staying.

Koepka, the U.S. Open champion, did not give a second thought to the fleeting encounter until his girlfriend used the car later that day. Upon returning, she said a man had approached to ask if she would persuade Koepka to sign his pin flag. The man matched the description of the fan Koepka had encountered earlier.

The incident, which occurred last week, before the Bridgestone Invitational here, unnerved Koepka. He wondered: How did the man know it was Koepka’s car, and what if the man had been carrying a knife or gun?

The threats facing the world’s best golfers were driven home at the British Open last month, when the rental home of the defending champion, Henrik Stenson, was burglarized while he was playing his first round. Someone broke a window in the backyard garden and made off with money, jewelry and much of his wardrobe.

Stenson said last week that the thief or thieves had not been caught, and that he was convinced he had been targeted.

In retrospect, he can see that he unwittingly left a trail of clues to where he was staying. He let Sky Sports film him walking into the rental house holding the Claret Jug. He took a photograph with fans in front of the house, and he parked his courtesy car, with Open Championship markings on the side doors, in the driveway.

“I guess it was a bit of an eye-opener in terms of when we’re at some of these big events, we do make easy marks for criminals who are quite clever at what they do,” Stenson said.

It used to be that the worst crime that players feared was the theft of bags from the trunks of their cars. The arrival of the internet and the escalation of prize money have upped the ante. The players’ competitive schedules are widely circulated, and at any PGA Tour stop, it is as easy to learn exactly what time the golfers are playing as it is to know when the buses or trains are running.

Their earnings also are a matter of public record, and with 36 players having earned at least $2 million this season, their wealth makes them attractive targets. Home addresses are at everybody’s fingertips, just a few clicks away, rendering the golfers’ bubble existence far from impermeable.

After a round at the Arnold Palmer Invitational in Orlando, Florida, in March, Jason Day said he received a call from his wife, Ellie, who had stayed at home in Ohio with the couple’s two young children. She told him that she had heard a prowler.

Day advised her to leave at once with the children, and he phoned a friend who is a police officer. The friend drove to the Days’ residence and, according to Day, found a man in dark clothes hiding in a tree on the property.

“Now if I’m gone, I have cops stay at the house,” said Day, who also plans to add a German shepherd to the family as a guard dog.

“What is really good is that the PGA Tour security does a fantastic job each and every week of trying to handle certain threats, and you don’t hear about them,” Day said.

The tour employs a director of corporate security, Steve Olson, and has a group of consultants, many of them private investigators or retired FBI agents, who work closely with tournament officials to ensure the players’ safety.

“Our long-standing policy is that we do not provide specifics on matters of security related to players and tournaments,” Laura Neal, vice president of communications, said in a statement.

The security team may keep a low profile, but its fingerprints are apparent in the new placement of the players’ parking lot here, near the first tee and protected by a makeshift barrier. This tournament and the Memorial outside Columbus two hours away — centrally located and with high-profile fields — are widely seen by players as events where autograph hunters, many of whom procure signatures to sell on the internet, pose the greatest hazard.

Bubba Watson, a two-time Masters champion, said someone had once followed him from the Muirfield Village course outside Columbus to his rental house after a charity event. Through a series of evasive turns, Watson managed to lose the person tailing him.

“I never go the same route to my hotel or my house,” Watson said. “I always change it up.”

He said he had also learned to eschew the tournament courtesy cars and instead rent vehicles at the airport. Or he rides with his caddie, Ted Scott. Watson does not post photographs on social media when he is out and about until after he leaves the spot.

“I’m weird,” he said.

Actually, security experts described Watson as wise. Chuck Tobin, president of the Association of Threat Assessment Professionals, an organization of law enforcement and private security officers who protect high-profile people, said recent studies had found that athletes, musicians and entertainers were targeted more often than chief executives, judges or entrepreneurs.

Tobin, who was the national director of security for Hillary Clinton’s 2008 presidential campaign, attributed the phenomenon to their globe-trotting lifestyles, and to the status conferred on them in a world that spins on a pop culture axis. Gone are the days when people with bad intentions collected information on celebrities by rooting through their trash, Tobin added. Now all they have to do is go through social media postings.

In a telephone interview, Tobin said, “One of the things I think is often overlooked is athletes’ lifestyles attract people, and their managers aren’t really equipped to give them the advice that they need to protect themselves.”

Tobin divided the “bad guys” into two groups: those who target an individual and those who take advantage of an opportunity, like a major sporting event where well-heeled athletes converge.

Rory McIlroy, who last played here in 2014, stayed at a different hotel this time. “The last couple of times I played here, autograph hunters checked into the same hotel,” he said.

Adam Scott, the 2013 Masters champion and former world No. 1, breathed easier when he saw a police officer patrolling the lobby of the hotel where he was staying, he said.

“That’s the first time I’ve seen that,” said Scott, who was playing in the tournament for the 15th time.

He is perhaps warier than most. Several years ago, he said, he enlisted the help of PGA Tour security when a stalker was pursuing him.

“Not to sound like a complete wuss,” Scott said, but the police presence “is quite comforting.”