Ask HN: How can we protect our source code/IP?

I don't mean ways to obfuscate the actual application code but, how can a company make sure employees do not bring the code home to later made UI changes and launch a copycat.

I have had all of my employees sign NDAs but what if they just copy the code and give it to a friend and s/he start a project using your IP?

Are there anyways to prevent this from happening?

What are google or facebook approaches to this risk?