He kept a sharp lookout for any strangers.

他警惕地守望着，不放过任何一个陌生人。

《牛津词典》

But in other points, as well as this, I was growing very lenient to my master: I was forgetting all his faults, for which I had once kept a sharp lookout.

但是在其他方面，如同在这方面一样，我对我主人渐渐地变得宽容了。

youdao

They uncovered "no smoking guns" responsible for the leak and consequently kept a sharp lookout for any icy flecks that might appear from the massive frame that holds the solar panels on the left side.

WSJ: Spacewalkers Replace Space Station Pump