Security Plans for When Your Senior Developer Leaves?

An anonymous reader asks: "Our CTO, responsible for all hardware and networking setup, who also coincidentally happens to be our senior (and only) developer, has just resigned to go work for the competition. We are not a software company, but he's written proprietary code that we use on a daily basis to work. What interim measures should we be taking to ensure a smooth transition to the next person hired to take over? What can we do about security, since this person designed and implemented all current security procedures? What about ensuring that we have all the intellectual property to which we're entitled? As one co-worker put it: 'His resignation was a surprise to us, but it definitely wasn't a surprise to him.' If he wanted to leave some hard-to-find malicious timed-release back-door-opening code running, it's certainly within his means. We don't expect any malicious action, and can rely on a reasonable level of co-operation and documentation before he goes, but I want to get a sense of what others have done in this situation."