North Korea has again sent balloons believed to be carrying rubbish across the heavily guarded border into South Korea, the Yonhap news agency reported on Wednesday. At least one of the balloons had landed on the grounds of the South Korean presidential office, Yonhap reported, citing the Presidential Security Service (PSS). “An investigation by the chemical, biological and radiological response team showed the objects did not present a danger or contamination, so they were retrieved,” the PSS said. Yonhap cited the South Korean Joint Chiefs of Staff (JCS) as saying that a number of balloons f…