the coast is clear

ＲＤ１０月号の記事、Where Has My Baby Gone?、赤ん坊を置いた車から一寸離れた隙に車を赤ん坊ごと盗まれた夫婦の話です。
Although Rudi and Katja are very happy that everything turned out well, they no longer feel safe at home since the thief has not been apprehended.
"He probably lives somewhere close by," says Katija. "It's very likely that car waiting for the coast to be clear."
最後に出てきた "the coast to be clear" は慣用句の様です。
辞書では "the coast is clear" で載っています。
・Cambridge Advanced Learner's English Dictionary: it is safe to do something or go somewhere because no one is watching or listening who would prevent you or catch you; You can come out now, the coast is clear.
・Wordnik:.the danger is over; no enemy in sight; I can wait there until I'm sure the coast is clear before I put my key into the thick silver lock and slip into the yard..
・Babyron.com: Is the coast clear? The coast is clear. There is no likelihood of interference. None of the coast-guards are about.
どうやら海からの密入国者が使った言葉から使われるようになった事が分かります。
便利な表現なので使えそうだ。（いつどこで？）