[anotheryou]

How do these large companies keep everything so secret (apart from "lost" prototypes)?

I'd think the press would acquire a trusty insider over the years. Alone keeping your work secret from your family would be difficult. Also I often hear "we people in the field knew for month/years" and wonder why nobody asked them before or if they just became so sure about something after the fact that it happened :)

[rdsnsca]

They leak fake secrets to suspected leakers and fire them if they show up in the news.

[anotheryou]

That only works to some extend. You'd need a different story for each suspected leaker and you can't fake what people are actually working on.

One strategy would be to subdivide Tasks in to small enough segments that you don't know what the big thing is you are actually working on. But this lack of overview might be very hindering/expensive.