Originally posted by Brother Stormhammer
There's a difference between launching a secret *payload* into orbit...which happens fairly frequently, and a secret launch (one that's totally unknown by the public). Even a relatively small (compared to a Saturn V) and common launch (something like a Delta, perhaps) will be a very noticed event. The launch itself will be visible (and audible) for literally miles, and the exhaust plume as the rocket climbs will be even more visible...not only from miles away on the ground, but to airline pilots and passengers and civil aviators over most of a state or two.

Air traffic controllers will know about it too...either because the FAA will notify them in advance (the usual method) or because of the sudden flurry of "What was THAT?!" traffic from the aforementioned pilots (in the event of some secret launch).

It's also far from trivial to hide a launch site. Too many very large buildings and fairly distinctive support structures are needed to ship / assemble / preflight / fuel / launch a large rocket.