Despite a bogey at the last, the ponytailed Siem captured only his second Tour title courtesy of a closing round 67 to win by a stroke at eight under par.
Italy’s Francesco Molinari stormed home in 64 to grab second place while French crowds cheered on Raphael Jacquelin (69), who finished third on six under par.
Molinari’s second place lifts him into an automatic qualifying position for the Ryder Cup with only seven weeks of the race to go.
World number three Lee Westwood climbed 18 places to 40th with a closing 70. That came a day after it looked as if he might be pulling out with knee and groin trouble after he slipped walking to the first tee.
“The leg is fine – it wasn’t a problem today,” Westwood said, after completing his final round before The Open at Royal Lytham starting on Thursday week.
Lawrie, who carded a last day 75 for an indifferent eight over par tally, was standing just off the opening fairway of the 2018 Ryder Cup host venue when a ball spotter informed him his tee shot had found a water hazard.
However, the Dubliner was of the opinion his ball had travelled further right and missed the water. But on the advice of the marshal Lawrie took a compulsory penalty drop. He then found his original ball but had to accept the eventual double bogey.
“It was one of the worst things that has happened to me on a golf course marshalling wise, ” said Lawrie.
Shane Lowry and Graeme McDowell shared the leading Irish honours in a share of 17th place and with Lowry carding a 70.
McDowell commenced his last round bogey, double bogey on route to 72.