Sharks have returned to Cape Cod just in time for the summer. At least two white sharks were detected by sensors off the coast of Chatham earlier this month, state officials said. “They were detected by acoustic receivers that our biologists placed in the water,” said Reginald Zimmerman, a spokesman for the Executive Office of Energy and Environmental Affairs. “They detected the two white sharks off Chatham around June 8.” The sensors, which are about a quarter-mile offshore, reacted to tags that marine biologists placed on sharks in previous years, Zimmerman said.