This is just an educated guess on my part, but lots of start-up companies use the melodramatic term "stealth mode" to refer to the period of development prior to their first public press release. "Low profile" might be a better term. They certainly do not have any way to remain truly invisible, but they try to minimize public chatter by not discussing revealing info and by telling their employees, friends, and business partners "Hey! Keep your mouths shut until we tell you otherwise."