BANGKOK—Thai authorities have ordered heightened security measures on the resort island of Phuket after the discovery of the body of a 57-year-old Swiss tourist amid signs of foul play, officials said Friday. The woman’s partially clad body was found face down in water in a rock crevice near a waterfall Thursday afternoon by an island resident, police said. From the condition of the body, it appeared she had been dead for several days, Phuket regional police commander Kitirath Phanpetch told local MCOT television. “From what we saw at the scene, the body was covered with a black sheet, which suggests it was done by someone and she did not die of natural causes,” he said. At a news conference in Bangkok on Friday, national police deputy spokesman Col. Kissana Phathanacharoen said investigators were still awaiting autopsy results to determine a cause of death. Government spokesman Anucha Burapachaisri said Prime Minister …