Zep 1:12 At that time I will search Jerusalem with lamps, and I will punish the men who are complacent, those who say in their hearts, ‘The LORD will not do good, nor will he do ill.’
In Zephaniah, God declares a coming day of judgement. In this depiction, God declares hyperbolically that He will “sweep away everything from the face of the Earth”. To illustrate the extent of the punishment, God describes how He will search diligently throughout Israel to identify those who are practical atheists (“God will not do good or evil”). The searching that is described is an active acquiring of knowledge through the mechanism of a search.
This language does not present an active omniscience of all events that God has in His mind from all eternity. Instead, like other verses in the Bible, God acquires knowledge from outside Himself. While the phrase is idiomatic, it is not a phrase that fits well with Classical depictions of Omniscience.