a misplaced sense of security. The building is not securing the source code. You should first focus your efforts on convincing why physical security of the building is not what is protecting the source. I ponder what extraordinary circumstances you might be working under, already. Are there not non-compete contracts in place with current employees?
Regarding hiring outside help, perhaps, there is another issue: assuming your company is in the US, is the code possibly subject to International Traffic in Arms Regulations (ITAR)? If there is a hint of a possibility, then you need to look into this as it will restrict who can be hired to work on the code, as well as the physical location.
I know of one company that was so distrustful of its employees (or, more likely trying to hide something) that only the founders were allowed direct access to the version control system for the flagship product. They had in place a ridiculous check-out, check-in procedure that slowed development, needless to say. It smelled pretty strongly that the source had been ripped off from the founders previous employer and that they worried that access to version control history would reveal that. No surprise that culture there was stifling and the guy I knew that worked there did not stay long.