I had a discussion the other day with a friend who claimed that many more cosmonauts were lost during the the Space Race than the Soviet government publicly admitted. He argued that it was easy for the USSR to hide their rocket launches and report only the successful ones because there wasn't a network of surveillance satellites yet, and there was no direct way that reports from local eye-witnesses would reach the West.
There is already a question on spaceSE about these "Lost cosmonauts". The consensus seems that these stories are either hoaxes or lack reliable evidence.
But I'm still wondering whether the claim of secret rocket launches is technologically plausible for that historical period. I think that there was much public participation in the early days of space exploration, with radio enthusiasts all over the world listening attentively to every signal transmitted from vehicles in orbit (this website hosts some historical audio recordings of radio signals from the Space Race, including recordings of Sputnik-1 and Explorer-1). And if amateurs did that, it's highly probable that both involved nations (USSR and USA) closely monitored all radio signals in order to gather as much information about their opponent's steps as possible (we know that this is what the ECHELON program did at least since 1972).
Yet, as I'm neither a radio nor a rocket expert I might be overestimating the difficulty of concealing a rocket launch during the early stages of space exploration. So, would it have been possible for the USSR (or NASA, for that matter) to launch rockets in secrecy at that time?