Student or Learner
'It's not good news, I'm afraid.' They all stared at him, waiting for the gate of the lion's cage to spring open 'It's probably has a wireless connection to a fail-safe device.'
Is it saying they wait for the man to say something bad, something they are afraid to hear?
It's not any sort of standard idiom I've ever heard before, or a standard metaphor, but you have understood it as I have - if the lion's cage springs open, the lion can jump out. Once the lion is out, it is dangerous.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.