They all stake their reputation so in theory they have checks in place to minimize rogue behavior (in practice though they are often clueless, see starfish). Also they put their code/hardware into public so they normally can't target individuals without fearing others to discover their attempts.
Contrast this to rogue employees at Github who can copy repositories for their old colleagues from college to give them an edge and the only difference you will ever see manifested is competitors seemingly always having timely answers to any new features you are about to unveil in your solution, because they were too smart to copy/paste your code directly.
If you even find out, and if you do it would cost a lot of time and money to reclaim what is yours. It's not hard to setup your own servers, it's becoming a lost art in today's cloud microcosm but its really not hard to do yourself and do it right.
You need to trust your server hosting company, your server OS vendor, your own workstation's software, your OS, all your firmware...
If GitHub employees stole proprietary code from you, and commercialized it, proving "prior art" and theft should be extremely easy for you. IANAL, but it's not something I'd worry about with most companies hosting on Github.