How did North Korean soldiers wander across the world's most heavily guarded border?

The Demilitarized Zone between the two rival Koreas might be the most heavily armed place on earth. So how, on the eve of Russian President Vladimir Putin’s summit Wednesday with North Korean leader Kim Jong Un, did as many as 30 North Korean soldiers wander over the line separating North from South, causing South Korea to fire warning shots before the North Koreans withdrew? The short answer appears to be shrubbery: Because of an overgrowth of foliage, the North Koreans may not have seen the signs marking the thin military demarcation line that divides the DMZ into northern and southern sides since the 1950-53 Korean War.