1、at midnight 是正确的介词短语形式，in midnight 是不正确的
因为：midnight 是午夜时分，即零点前后。
所以：按日期时间的介词使用规则而言：in 明显不合适了哈。
Whoever in the forest is hardly seen at midnight, especially the person with a dark green coat.
2、但 in the midnight 则也是正确的。
He was seen going quickly through that lane in the midnight yesterday, so that the cop suspects him isn't surprising at all.