Unless you're working with something highly classified and given that you can't block email and USB devices I guess you aren't there's really not to much damage to be had even if the source code leaks. The thing is, what is the code, or parts of it worth without the knowledge of how it works and the organization around it.
In general the value of "source" is much less than is commonly touted, basicly the source without the people or the organization isn't worth the storage it occupies for a competitor.
Also, you're missing the most likely attack vector, and it's also the one you can't stop no matter what. If someone really really want's to know how you made your magic then they'll try to hire your developers away, and since you can't stop them from having information inside their skull and even if they turn in all their possesions ther knowledge and domain expertise is leaving with them. Basicly employee retention and trust is the only way. Sorry.
