That statement is often used as a threat. It doesn't mean much if the person saying it is simply talking a good game and not going to do anything further.
It's a whole different story if someone says that and is completely sold on the idea of acting on that knowledge.

It's a great day when God says, "I know where you live!" to your greatest enemy.
You can rest assured that God never breaths out idle threats...only promises backed with action.

The prophet Isaiah tells a story of this happening in Israel's history.

The powerful king of Assyria named Sennacherib starts running off at the mouth taunting Hezekiah, Israel's King. Hezekiah trains his people not to respond to these threats coming from his messengers. (Isaiah 36:21)

God's response to Sennacherib in Hezekiah and Israel's defense is priceless...and down right scary... "But I know where stay and when you come and go..." (Isaiah 37:28) You've got the love the Bible....God tells this feared and powerful king, back down big boy because I know where you live!

As we look to the Lord He has a habit of fighting our greatest enemies for us. He may wait until it looks like there is no chance for our victory before intervening many times, but rest assured and draw strength from these words...