on the prowl

(be/go) on the ˈprowl
(of an animal or a person 人或动物) moving quietly and carefully, hunting or looking for sth 悄然潜行（以捕猎或寻找）
There was a fox on the prowl near the chickens.
有一只狐狸在鸡群附近徘徊。
an intruder on the prowl
一个蹑足潜入的人